The effects of process parameters of biomass grain size (ranging from 0.425 to 3.35mm), rotational speed (ranging from 20 to 60rpm) and the pyrolysis temperature (ranging from 400 to 550 oC) on the yield of bio-oil were investigated in this study by using a fast pyrolysis reactor with a single tapered screw extruder. This study gives the optimal pyrolysis temperatures and rotational speeds for different grain sizes to achieve peak bio-oil yield. The results indicate that higher feed rates are required for larger grain sizes to achieve peak bio-oil yields. For instance, a 20rpm rotational speed is for 0.425~1.18mm grain sizes (50~57% bio-oil yield), while a 40rpm rotational speed is for 1.7~3.35mm grain sizes (56~60% bio-oil yield). This implies that the productivity of bio-oil can be significantly increased in the current system. For the case of 2.5~3.35mm grain size, which corresponds to an optimal rotational speed of 40rpm, the production rate was estimated to be around 4kg/h.