A three-dimensional numerical model was developed for a two stage entrained flow coal gasifier. In the model, the coal devolatilization, volatile and char combustion, char-H2O and char-CO2 gasification, and gas-phase shift reaction were taken into account in accordance with the actual process. Four mixture fractions were used to simulate the coal gasification. The influence of turbulence on gas composition was taken into account by a pdf model with a clipped Gaussian distribution function. The model uses conventional numerical methods and submodels to calculate gas and particle temperatures, gas and particle velocities, mean turbulent kinetic energy and turbulent energy of dissipation, gas species composition, particle trajectory, extent of reaction, and radiant heat flux. The effect of mixture fraction fluctuations on the overall gasification characteristics due to turbulent flow was investigated for gasification in an IGCC gasifier. It was revealed that the fluctuations of mixture fractions of gases produced from devolatilization and char-O-2 reactions have important influences on not only distributions of temperature and gas composition, but also cold gas efficiency and heating value of product gas. However, the fluctuations of mixture fractions of gases produced from char-CO2 and char-H2O reactions have only a limited effect. Including the fluctuations of mixture fractions smoothens the distribution of temperature. However, the fluctuations of mixture fractions concerning all the reactions have little effect on the distribution of particle concentration. For gasification in an entrained flow IGCC gasifier, with little influence on the validity of the simulated results, the neglecting of the fluctuations of mixture fractions of gases produced from char-CO2 and char-H2O reactions can significantly shorten the calculation time. However, the fluctuations of mixture fractions of gases produced from devolatilization and char-O-2 reactions cannot be neglected. Comparison between prediction and measurement suggests that including fluctuations of mixture fractions gives a better prediction.