Abstract: Effects of 'trifluralin' as herbicide on weed spectrum and yield of canola were investigated in the field under rainfed conditions. Four application rates of herbicide were used as pre-plant soil incorporation. This study was carried out at the National Agricultural Research Centre (NARC), Islamabad for two growing seasons. Grassy, sedges and broad-leaf weeds were observed in the experimental field. Trifluralin applied @ 1.5 L. a.i. ha-1 and 2.0 L. a.i. ha-1 simultaneously had significant effects on total weed spectrum (density + biomass) of canola crop in both the growing seasons. Amongst different application rates of trifluralin, 1.5 L. a.i ha-1 produced the highest seed yields of 1968 kg ha-1 that was significantly (p = 0.05) higher than rest of the application rates during 1999-00. In 1998-99 growing season, the highest canola-seed yield of 1776 kg ha-1 was also produced by 1.5 L. a.i. trifluralin ha-1, which was remained at par with 1.0 L. a.i. trifluralin ha-1. Both seed weight and seed yield of canola for the second season (1999-00) was higher than the first season (1998-99). However, the highest application rate (2.0 L. a.i. ha-1) of the herbicide in both the seasons caused reduction in seed yield of canola that would be due to its phytotoxic effect on canola plants growth.