A field experiment was conducted for two summer reasons to identify critical period in irrigated summer sown blackgram (Vigna mungo L.) at Kanpur. Unchecked weed competition throughout the crop growth period caused a reduction of 40.10% in seed yield. Initial weed condition for the first 10 days after sowing did not cause significant loss in yield. Weed-free conditions maintained 40 days after sowing could not bring significant increase in seed yield. The critical period of crop-weed competition thus lies between 1040 days after sowing in irrigated summer sown blackgram.