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 degrees 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 degrees C, are 8.20-14.6 vol% (H-2), 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% (H-2), 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/Nm(3), 1.50-1.95 Nm(3)/kg, 52.2-75.9%, and 30.9-56.4% for CS, respectively, and 3.82-5.09 MJ/Nm(3), 1.48-1.92 Nm(3)/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. (C) 2019 Elsevier Ltd. All rights reserved.