Torrefaction is a thermal pre-treatment process to pre-treat biomass at temperature range of 200-300°C under an inert atmosphere. It was known that torrefaction process strongly depended on the decomposition temperature of the lignocellulosic constituents in biomass namely hemicellulose, cellulose and lignin. In this study, the torrefaction behaviour of Empty Fruit Bunches (EFB), Palm Mesocarp Fiber (PMF) and Palm Kernel Shell (PKS) were investigated. The study focuses on the relation between the lignocellulosic constituents with torrefaction process. Two different size ranges of 250-355 μm and 355-500 μm were used and then submitted to six final torrefaction temperatures of 200, 220, 240, 260, 280 and 300°C. The process was carried out in a thermogravimetric analyzer coupled with mass spectrometry (TGA-MS). The results implied that torrefaction was strongly depended on the thermal decomposition behaviour and composition of lignocellulosic constituents. The ultimate analysis showed that torrefaction increased the carbon content of torrefied solid, whilst decreased the hydrogen and oxygen content. Based on decomposition temperature, the TGA curves indicated that hemicellulose was more significantly decomposed than cellulose and lignin during torrefaction. Due to higher content of hemicellulose in EFB compared to others, EFB had been decomposed almost completely by torrefaction. From the mass spectrometry study, the percentile compositions of CO, CH 4, CO 2 and H 2 in the gases product were found to be 29-33, 20-23, 1.3-1.9 and 1.7-2.1, respectively. It can be concluded that torrefaction of oil palm biomass affected by the chemical composition and decomposition temperature of hemicellulose, cellulose and lignin. © 2012 Asian Network for Scientific Information.