Field studies were carried out to determine the critical period of crop-weed competition in blackgram (Phaseolus mungo). Trianthema portulacastrum, Digera arvensis, Echinochloa crusgalli, Parthenium hysterophorus, Phyllanthus niruri and Cynodon dactylon were the most predominating weeds. Grain yield loss increased with the increase in the duration of competition and maximum loss (67%) occurred due to full season competition. Significantly higher grain yield (12.42 q/ha) and yield attributing characters were obtained in plots remaining weed free upto harvest. The critical period of weed competition was between 30 to 45 DAS during which the crop should be kept free of weeds to prevent the potential loss in blackgram grain yield.