Fast pyrolysis was used to convert waste biomass into bio-oil, which has a benefit of storage and transportation with the potential as a fossil oil substitute. Pakistani cotton stalk was pyrolyzed in a bench-scale bubbling fluidized bed reactor. The effect of reaction conditions such as temperature and feed size on the bio-oil, char and gas yields was investigated. The optimal pyrolysis temperature for the production of bio-oil was 490 • C which gave the maximum yield (36 wt%) of product at feed size of 1.0 mm. Bio-oil yield increased with the increase in temperature, while the yield of char decreased. The various properties of bio-oil attained under these pyrolysis conditions were defined. Chemical composition of bio-oil was determined using FTIR and GC-MS analysis, and major chemical compounds were phenols, carboxylic acids, ketones, aldehydes, furans and sugars.