Optimizing the effectiveness of herbicides by additives is one of the ecological and economical approaches for weed management. The aim of this study was to investigate the effect of vegetable oils on the efficacy of clethodim herbicide in control of foxtail as a factorial based on a completely randomized design including clethodim concentration in six levels (0, 7. 5, 15, 30, 60 and 120 g ai per ha) and additives in eight levels (without vegetable oil and vegetable oils of sesame, bitter almonds, sweet almonds, cottonseed, canola, rapeseed and soybean) with four replications in 2014 in Research Greenhouse of Faculty of Agriculture, Ferdowsi University of Mashhad, Mashhad, Iran. The results of this study showed that clethodim concentration, vegetable oil types and interaction effect of concentrations in vegetable oils had a significant effect on measured traits of foxtail including height, fresh and dry weight of shoot and root and root volume. Increasing relative potency for all traits measured based on 50% effective dosage was observed, which indicates an increase in the efficiency of clethodim in foxtail control. The results of this study also showed a 90% effective dose reduction for fresh and dry weight of green foxtail in the presence of all vegetable oils. The higher relative potency for fresh and dry weight of roots compared with fresh and dry weight of the shoot showed greater root susceptibility to the application of vegetable oils by increasing the transfer of clethodim to the root. The application of sesame, bitter almonds, sweet almonds, cottonseed, canola, rapeseed and soybean oil led to 1. 20, 1. 26, 1. 42, 1. 39, 1. 41, 1. 43, and 1. 41 times increases, respectively, in clethodim performance. Finally, application of vegetable oils as additives to clethodim herbicide can be considered as an ecological and environmental approach for controlling green foxtail.