Abstract: Determining the effect of different sources of nitrogen and weeds on two varieties of soybean, an experiment was conducted out at research station of Tabriz University, Iran. Treatments were two varieties (Williams and Harcor), four nitrogen levels (two levels of symbiotic bacteria and two levels of urea) and weeds (with and without control). Results showed that three various factors can significantly change the Photosynthetically Active Radiation (PAR) amount at different layers of canopy. Solar radiation decreased in a vertical profile of canopy because of increasing Leaf Area Index (LAI). At the bottom level of canopy only 6% of total arrived PAR was recorded. Increasing LAI caused a sharp decrease in Light Interception Efficiency (LIE) in the canopy. Weeds significantly affected root nodules in dry and fresh weights. So, in weedy condition of soybean canopy, accumulation of dry matter decreases due to decline in LIE. On the other hand, weeds can strongly affect on nodules activity and decrease potential of nitrogen fixation by symbiosis bacteria.