During gas nitriding process, homogeneous velocity distribution is required to ensure uniformity of gas content and temperature fields, which is one of key factors for fine processing results. In order to understand the velocity distribution in a gas nitriding furnace, which was equipped with a stirring motor-driven fan on top, we studied different situations with several fan motor rotational speeds. The whole furnace was modeled by computational fluid dynamics software package FLUENT. The fan boundary condition and standard k-ε equation are chosen for modeling to generate better results in our case. A digital turbine anemometer is used to measurment velocities at various places inside the furnace. Good agreements were found between simulation and measurement.