Gasification of coconut shell (CS) and palm kernel shell (PKS) is conducted in a batch type downdraft fixed-bed reactor to evaluate the effect of particle size (1�3 mm, 4�7 mm, and 8�11 mm) and temperature (700, 800, and 900 °C) on gas composition and gasification performance. The response surface methodology integrated variance-optimal design is used to identify the optimum condition for gasification. Gas composition, which is measured using the biomass particle size of 1�11 mm at 700�900 °C, are 8.20�14.6 vol (H2), 13.0�17.4 vol (CO), 14.7�16.7 vol (CO2), and 2.82�4.23 vol (CH4) for CS and 7.01�13.3 vol (H2), 13.3�17.8 vol (CO), 14.9�17.1 vol (CO2), and 2.39�3.90 vol (CH4) for PKS. At similar conditions, the syngas higher heating value, dry gas yield, carbon conversion efficiency, and cold gas efficiency are 4.01�5.39 MJ/Nm3, 1.50�1.95 Nm3/kg, 52.2�75.9, and 30.9�56.4 for CS, respectively, and 3.82�5.09 MJ/Nm3, 1.48�1.92 Nm3/kg, 59.0�81.5, and 33.0�57.1 for PKS, respectively. Results reveal that temperature has a greater role than particle size in influencing the gasification reaction rate. © 2019 Elsevier Ltd