Abstract 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.