The objective of energy usage and benefit-cost analysis of Bt cotton was to determine the energy efficiency indices under different weed management practices in Bt cotton. The field experiment was carried out at college farm, Professor Jayashankar Telangana State Agricultural University, Rajendranagar, Hyderabad during Kharif, 2014 with ten weed management practices. The inputs in the calculation of energy usage in agro-ecosystems embraced labour, machinery, electricity, diesel oil, fertilizers, herbicides, pesticides, seeds, while seed cotton was included in the output. The results depicted that total input and output energy under different weed management practices were about 16051 to 18550 and 2360 to 16838 MJ/ha, respectively. Out of all the treatments tested the highest energy use efficiency (0.9), energy intensiveness (0.7 MJ/), specific energy (12.9 MJ /kg), energy productivity (0.9 kg/MJ), net energy       (-1712 MJ/ha) of Bt cotton production system were reported in mechanical weeding thrice at 20, 40 and 60 DAS. However the lowest energy intensiveness of 0.5 MJ per rupee was reported in pre emergence application of pendimethalin at 1000 fb 2 HW at 20 and 50 DAS treatment.