The uniform temperature and mass current density of gas in deposition regions of films were very important to obtain uniform diamond films over a large area during hot filament chemical vapor deposition. The simulation showed that the two physical parameters of gas in the chamber varied as a function of space and were influenced by the arrangement of the filaments and the initial rate of the introduced gas. Regions of uniform temperature and mass current density existed in the chamber. The distributions of temperature and mass current density did not vary with the filament diameter. However, the value of the two physical parameters increased and declined with the increment of the diameter, respectively. An optimum distance between filaments was necessary to obtain uniform films over a large area. The mass current density increased with the initial rate of the introduced gas. These results might provide a basis for optimizing the technological parameters to obtain uniform diamond films over a large area.