Mustard (Brassica juncea L.) is the main crop among the oilseed crops of the world. Weeds are the main factors, responsible for the low production. Field research was planned and conducted during the winter season of 2016-17 at the College of Agriculture, the University of Sargodha to evaluate the impact of various row spacing on yield and its components under various weed competition durations. Maximum weed dry weight (79.1g m-2) was found at weedy condition till harvest while lowest (9.1 g m-2) in weed-free conditions till 70 DAS excluding the weed-free conditions. A higher number of branches plant-1 (7.8), no. of plants (137), 1000-seed weight (3.3 g), seed yield (1536 kg ha-1) was in narrow 15 cm row spacing over the wider row spacing. About 42.64% yield reduction was recorded in weedy conditions till harvesting over weed-free conditions up to harvesting while 9.34% yield reduction was recorded when row spacing was increased from 15 cm to 20 cm while a 10.27% yield decline observed when row spacing increased from 20 cm to 25 cm. The critical period for weed control is 38DAS. The interaction was also significant. Maximum seed yield (1965.7 kgha-1) was found at 15 cm row spacing with weed-free conditions till harvesting followed by (890 kg ha-1) at 25 cm row spacing with weedy conditions till harvesting of the crop. It is concluded that as the row spacing of the crop increases from narrower to wider, the weed dry matter increases and seed yield decreases.