Imazethapyr is a now emerging post-emergence (PoE) herbicide for pulses. A field experiment was conducted for two consecutive years (2011-12) to assess the efficacy of imazethapyr in summer mungbean (Vigna radiate L.) in a mild alkaline soil of Kanpur, India. Treatment comprised of five different doses of Imazethapyr [0 (Weedy check), 40, 60, 80, 100 g ha-1] along with weed free treatment. Results revealed that summer mungbean yield was reduced up to 38.6% due to weeds infestation. Post-emergence application of imazethapyr significantly reduced the (P£0.05) weed population and weed biomass and the herbicide effectiveness was increased up to the dose of @ 80 g/ha. The relative suppression effect of imazethapyr was in the order of grassy weed>Sedges>broad-leaf weeds. However, the study suggest that the dose of 100 g/ha is needed for suppressing the Cyperus spp. to a significant extent. The weed control efficiency (WCE) of imazethayr was more than 40 per cent at a dose of 100 g/ha and the corresponding value of weed control index (WCI) was more than 60 per cent. The highest average grain yield of summer mungbean was recorded in weed free plot (926 g/ha) which was on a par with imazethapyr @ 100 g/ha (920 kg/ha) and imazethapyr @ 80 g/ha (883 kg/ha). The increase yield under imazethapyr treated plots was mainly due to the increase in plant biomass, nodule number, root weight, number of seed per plant and number of pod per plant. Although, the net return was highest under imazethapyr @ 100 g/ha (Rs. 24, 880/ha) the marginal rate of return was higher in imazethapyr @ 80 g/ha (627%).