In order to study light COMPETITION in mixed canopy of wheat and weeds, two experiments were conducted in Ferdowsi University of Mashad, Iran during 2000-2001 cropping season (field study), and Guelph University, Ontario, Canada (2002), (growth cabinet study). The treatments of the field study included three levels of Cruciferous weeds (wild mustard, turnip weed and flix weed) and five levels of weed density (0,4, 8, 16 and 32 plants/m2 for wild musturd and turnip weed, and 0, 16, 32, 64 and 128 plants/m2 for flix weed). Randomized complete block design with four replications in an additive series technique with fixed no. of 450 wheat per square meter was employed as the experiment design. In growth cabinet study, is used a model system of annuals to examine how canopies of species having differing morphologies differed in light-interception. Wheat, wild mustard and flix weed were grown as "targets", surrounded by neighbours of a single species. Neighbours could be anyone of the target species. Plants were grown in pots, with one target plant and three neighbors plants. Results of experiment indicated that wild mustard and turnip weed (in comparison to flix weed) had more effect on PPFD and R: FR. In addition, with increase in leaf area of mixed canopy, the quality and quantity of light changed.