Field experiment was conducted to study the effect of post emergence herbicide imazethapyr on weed infestation and yield of black gram variety (ADT 3) under rice fallow situation (relay cropping). The treatments consisted of three times (14, 21 and 28 days after sowing) of application in main plot and post emergence herbicide imazethpyr at 60, 75 and 90 g/ha and an absolute control formed the subplot treatments. The effect of imazethapyr on weed density, weed dry weight and weed control efficiency was at par when applied on either 21 or 28 DAS. Imazethapyr of 90 g/ha recorded lowest weed density and weed dry weight and was at par with that of 75 g/ha. Higher growth and yield attributes of black gram were associated with imazethpyr application on 21 DAS at 90 g/ha. The highest grain yield of 759 kg/ha was recorded under 21 DAS than other times of application of herbicide. Among the doses, application of imazethapyr at 90 g/ha registered significantly high grain yield (751 kg/ha) in comparison of other doses and control. The favorable economic benefits in terms of higher gross income, net income and benefit cost ratio was high by the application of imazethpayr at 90 g/ha on 21 DAS