This research was carried out as a split plot experiment based on a randomized complete block design with three replications at Agricultural Faculty of Bu-Ali Sina University, during 2008-2009. Experimental treatments were amounts of nitrogen fertilizer at four levels (100, 150, 200 and 250 kgN.ha-1) and five wild mustard densities (0, 4, 8, 16 and 32 plants.m-2). Nitrogen fertilizer increased canola biological and economic yield. In whole nitrogen levels, increasing of wild mustard density and dry matter reduced biological and economic yield of canola. Also, in whole evaluated densities, wild mustard dry matter enhanced by more of nitrogen application. By increasing nitrogen up to 200 Kg N ha-1, the initial slop of biological and grain yield loss of canola reduced in compared with plant density and dry matter of wild mustard. Wild mustard seed production variation in response to its plant density and dry matter showed a non-linear trend and enhancing of nitrogen application increased initial slope of wild mustard seed production. By increasing wild mustard plant density in each of four nitrogen levels, population growth rate of weed was reduced and there wasn’t very difference between 200 and 250 Kg N ha-1. Dry matter of weed than its plant density was a better indicator to estimate reduction of biological and grain yield of canola and wild mustard seed production. According to obtained results, 200 Kg N ha-1 is recommended for Hamedan and similar regions.