Biomass oxygen-steam gasification associated with synthesis technology known as indirect biomass liquefaction is regarded as one of the most promising technologies of biomass utilization. In this paper, a comprehensive gasification model was developed for the simulation of rice straw oxygen-steam gasification using ASPEN PLUS. The gasification process was divided into two parts: pyrolysis and gasification. The RYield module was used to simulate the pyrolysis process with an external FORTURN program to calculate the pyrolysis products while the gasification process was calculated by the RCSTR module. With the help of the model, the gasification of rice straw was simulated under different residence time, different temperature and different amount of steam. The results showed that the proper residence time and temperature is 1.5s and 1300°C, respectively. The optimum amount of steam is steam/biomass=0.12 while the addition of oxygen is oxygen/biomass=0.2.