In the gasification of an aquatic biomass with He/CO sub(2)/O sub(2), the effects of the concentration of CO sub(2) and O sub(2) in the gasifying agent and the feeding rate on the gasification behavior were investigated using a downdraft fixed-bed gasifier at 900 degree C. Using CO sub(2)/O sub(2) as the gasifying agent led to an increase in the conversion to gas and the syngas (CO + H sub(2)) content because the gasification of char with CO sub(2) (C + CO sub(2) arrow right 2CO) and the decomposition of tarry compounds were promoted. Increasing CO sub(2) content led to the increase in the conversion to gas and CO content and the decrease in the H sub(2) content. With increasing O2 content, contents of CO and H sub(2) increased while the conversions to gas remained almost constant. Especially with CO sub(2)/O sub(2) = 45/55 vol.%, the conversion to gas was 94.0 C-mol% and the syngas content exhibited a maximum value of 69.7 vol.%. As the feeding rate was decreased, contents of CO and H sub(2) decreased while the conversion to gas remained almost constant. The nitrogen atoms in the feedstock were mainly converted to form N sub(2). H sub(2)S and COS were the main sulfurous gases. The sulfur content in the char was much higher than that in the feedstock.