Biomass co-gasification is among the most encouraging bio-conversion technologies used in the production of clean and sustainable fuel. This research work was performed on the co-gasification of oil palm trunk and frond to explore the mutual relation of input variables on syngas and optimize them for maximum syngas generation. Air was utilised as the agent of gasification, while a downdraft gratifier was used as the reactor. The operating variables are temperature (700�900 °C), blending ratio (20�80 wt) and particle size (1.18�4 mm). Optimization was achieved through Response Surface Methodology, Box-Behnken design. The results showed that the operating variables influenced the results in increasing order of temperature, particle size, and blending ratio. Optimum syngas composition was obtained as 20.22 vol H2, 24.86 vol CO, and 13.78 vol CH4 at a temperature of 900 °C, particle size of 2.59 mm and a blending ratio of 20OPT:80OPF.The optimized values resulted in a high syngas yield and co-gasification performance. Cost analysis of the optimum blend was carried out, and it was found that the operating cost played a significant role in the final syngas cost. It occupied about 49 of the production cost, while the capital and other cost occupied occupy 44 and 7 respectively. © 2021 Elsevier Ltd