A field experiment was conducted during kharif 2007 at Tamil Nadu Agricultural University, Coimbatore to study the effect of different densities of Trianthema portulacastrum on growth and yield of cow pea. Plant height, leaf area and dry matter production of cow pea was reduced with increase in weed density from 0 to 64 weeds/m2 in all growth stages. The infestation of 16 weeds/m2 of Trianthema portulacastrum significantly decreased the dry matter production of cow pea (36.3 g/plant) compared to zero weeds/m2 (64.5 g/plant). The maximum yield reduction (34.8%) was observed with 64 weeds/m2. It was concluded that the population of 8 weeds/m2 of T. portulacastrum is the threshold value for cow pea