Thermochemical conversion process by the gasification to convert the agricultural residues to gaseous fuel using the biomass partial oxidization theory is a very potential as a simple, clean and sustainable method for producing heat and power at rural and remote regions with less contamination compared to the fossil fuel. The biomass gasification process produced a mixture of gases called the producer gas, which contains of carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), hydrogen (H₂) and nitrogen (N₂). Hence, the aim of this investigation is fabricating and evaluating  the performance of a prototype fixed bed downdraft gasifier  using corn stover pieces and air as a gasifying agent under different equivalence ratios (ER) of  0.18, 0.24 and 0.36, corn stover particle size (PS) of 1-5, 6-10 and 11-15 mm and working temperature (WT) after throat of 700, 800 and 900°C, takes into consideration the gas composition, temperature profile, the lower heating value, gas yield and gasification efficiency. According to the obtained results, it is recommended to operate the gasifier  at  ER of 0.24, PS of 1-5mm and working temperature of 900°C, obtained the highest  concentration for CO (18.02%), H₂ (11.22%), CH₄ (3.317) and C_nH_m (1.751%), gas lower heating value of 5.833 MJ/Nm³ at gasification efficiency of  66.89% and gas yield of 1.70 Nm³/kg.