A field experiment was conducted during kharif 2011-12 and 2012-13 to study the bio-efficacy of herbicides in Bt cotton. Fifteen weed control treatments were tested in randomised block design and replicated thrice. All the herbicide treatments significantly reduced dry weight of weeds over un weeded control. Among the herbicide treatments sequential application of pre emergence application of pendimethalin 38.7% CS @ 0.68 kg ha-1 fb pyrithiobac sodium 10% EC 0.125 kg ha-1 or quizalofop-p-ethyl 5 EC @ 0.05 kg ha-1 as post emergence  were quite effective in controlling weeds which reflected in significantly decreased weed density, weed dry weight and increased weed control index. These herbicide treatments also recorded significantly higher seed cotton yield (3015 and 2916 kgha-1, respectively). Sympodial branches plant-1, dry matter production plant-1, number of bolls plant-1 and boll weight (g) followed similar trend as that of seed cotton yield. The weed index was also lower in the above treatments (5.89 and 9.00 %, respectively) compared to un weeded control (79.27%). Gross returns, net returns and B:C were significantly higher in these treatments as compared to other herbicide treatments.