Introduction: Weeds control has been recognized as an essential action in crop production systems. Weeds, in addition to crop quantity, are effective on crop quality, harvest costs and pest abundance diversity. Onion (Allium cepa L. ) cannot tolerate prolonged competition withweeds because of its initial slow growth rate after planting, shallow fibrous roots, and small above-ground canopy for effective soil shading (7, 8). Weed infestation in garlic is one of the major factors for loss in yield and bulb loss to the tune of 40-80% (2, 5, 25). Regarding the increase and development of onion cultivation in recent years, effective control of weeds in all stages of growth stages is necessary in order to increase bulb yield. In all researches, hand weeding has the highest onion yield, but due to increased production costs, it may not be economies. The use of herbicides can be an alternative to lowering costs, lack of labor and increasing onion gain. Herbicides such as Oxyflourfen, Oxadiazone, Linerone and Pendimethalin are used to control onion weeds (11, 15). With evaluation the effect of Oxyflourfen and Oxadiazonon onion weed in the early growth stage, it was demonstrated that the Oxyflourfen after hand weeding had the best weed control and onion bulb yield (7). In general, very limited research on chemical control of onion weeds in Iran and in the world has been carried out. Therefore, this research was carried out to evaluate the effect of three post-emergence herbicides in two application timing on weed control and onion bulb yield in Ramhormoz region. Materials and Methods: In order to evaluate the effect of application time of some post-emergence herbicides on onion (Allium cepa L. ) weeds control, a factorial experiment was conducted in complete randomized blocks design with 3 replications in Ramhormoz region. Experimental factors included post emergence herbicides (Clethodim 1. 5 Lit. ha-1, Oxadiazon 2 Lit. ha-1, Oxyflourfen 1. 5 Lit. ha-1 and Clethodim+Oxadiazon 1. 5 and 2 Lit. ha-1, respectively) and two different times of herbicide application (1-2 and 3-4 true-leaves stage of onion). Onion seeds were planted directly on Oct. 4th and the onion planting density was 25 plants per m-2. Fields received 100 kg P ha-1 and 150 kg N ha-1. The P was supplied in the form of single superphosphate. Nitrogen treatments were applied as urea (46% N). Dominant weeds in time of herbicides application were Echinocloacrusgalli, Malvaspp, Convolvulus arvensisand in time of bulb formation were Malvaspp, Melilotusofficinalis and ScorpiurusmuricatusL. . Living weed plants were calculated before and after herbicide application using 0. 5×0. 5m quadrate, and weed biomass obtained by harvesting and drying tissues at 75 ˚ C to constant mass using 0. 5×0. 5m quadrate at bulb formation stage. In order to evaluatethe efficiency of weed control, below equation was used: WCE=[(A-B)/A]×100 In this equation, WCE is the weed control efficiency (reduction of weed number in percent), A is weed density before and B is the number of weeds after herbicides application. The evaluated onion traits included: bulb fresh weight, bulb number, bulb diameter, bulb dry weight. Statistical analysis was made using the SAS statistical program. Differences between traits means were assessed using Duncan's Multiple Range Test. Results and Discussion: The major weeds flora in the field experiment were Echinochloa crus-galli., MalvarotundifoliaL. and Convolvulus arvensisL. Results showed that the effect of time of herbicides application on evaluated traits was not significant but the effect of herbicides treatments on the decrease of weed density and biomass was significant. The lowest Echinocloacrusgalli abundance and the highest control of this narrow weed was obtained from Clethodimherbicide treatment. Clethodimdid not control broadleaf weeds. Regarding the growth and coincidence of broadleaf weed growth such as Scorpiurusmuricatus L, Malvaspp and Melilotusofficinalis with onion growth, Clethodimhad no effect on these weeds and had the lowest onion yield. Oxyflourfen was the most effective herbicide onthe decrease of weed density and dry biomass (including Malvaspp and Melilotusofficinalis)and resulted in the highest bulb fresh yield, the bulb number, the bulb diameter and the bulb dry weight. It has been reported that the use of Oxyflourfen during post-harvest resulted in the highest number of onion bulbs per unit area, and then Oxadiazone and Fluozifop were arranged in subsequent positions (13). Although the difference of dry weight of bulb was not significant between different application timing, post-emergence application of oxyfluorfen at 3– 4 leaf stage resulted in onion. Thus, itwas recommended for effective onion weed control and high economic yield.