Abstract: The flow field in metal organic chemical vapor deposition (MOCVD) reactor is simulated three-dimensionally by computational fluid dynamics (CFD) theory. The governing equations are discretized with the finite volume method based on the non-stagger grids system and are solved by the SIMPLE scheme and the improved pressure-velocity method. How the different working parameters affect the flow patterns in the MOCVD reactor is studied in this paper. The geometric structure and inflow pattern of a MOCVD reactor which is under trial-production are improved in order to provide steady and uniform flow field and improve the quality of GaN films.