Torrefied biomass is a commercial fuel, which is particularly produced from woody biomass via torrefaction and alternative to coal to reduce greenhouse gas emissions in the short term. In this study, torrefaction conditions of cotton stalk were optimised, and the effect of bulk density together with temperature and residence time on process yields and characterisation of torrefied cotton stalk was investigated. Response surface methodology using Box-Behnken design was employed for the design of experiments and optimization. Cotton stalk was torrefied at a temperature between 260 degrees C and 320 degrees C, in 10, 35, and 60 minutes with bulk density of 125, 150, and 175 kgm(-3). Temperature was the most effective parameter for the six responses (higher heating value, carbon content, hydrogen/carbon and oxygen/carbon ratios, mass loss, and energy yield). Besides, temperature/bulk density interaction was found to be significant for all responses whereas residence time/bulk density interaction was effective for process yields. The optimization results showed that more economical torrefied biomass with similar quality of lignite could also be produced under process conditions of 305 degrees C-32 minutes-158 kgm(-3). At this point, higher heating value and carbon content of product were calculated as 19.7 MJ kg(-1) and 64%, respectively.