Field emission characteristics have been investigated for P-doped polycrystalline diamond films. It is demonstrated that the turn-on voltage of the electron emission decreases with increasing temperature for the P-doped diamond film, while no variation in the turn-on voltage occurs for the undoped diamond film. The temperature-dependent field emission characteristics are found to be inherent to the P-doped diamond film. A behavior of the field emission characteristics can be well explained by means of the thermionic field emission model combined with the temperature dependence of the ionized donor concentration. This means that an increase of the ionized donor concentration with increasing temperature may lead to a reduction in the tunnel barrier width at the interface between the diamond and the cathode, resulting in an enhancement of the internal emission current. It is suggested that the internal electron emission is important to the field emission characteristics of the P-doped diamond films. A variation in the field emission characteristics of P-doped diamond films with various cathode metals and with various P-doping concentrations can be consistently understood on the basis of the internal electron emission model.