Electron emission characteristics have been investigated for polycrystalline diamond films subjected to H-2 plasma treatment, O-2 plasma treatment, and annealing. In the case of surface treatments of diamond film, the voltage commencing electron emission increases due to O-2 plasma treatment or annealing and is recovered by the subsequent H-2 plasma treatment. On the other hand, in the case when the rear face of the diamond film is treated with O-2 plasma or annealed in diamond/Au samples, no recovery of the voltage occurs due to H-2 plasma treatment after O-2 plasma treatment or annealing. It is suggested that there exists another factor dominating the field emission characteristic other than field emission mechanisms such as electron injection at the diamond/metal contact and electron emission at the surface with negative electron affinity.