The field emission properties of a polycrystalline diamond film with hydrogenated and hydrogen free surfaces have been investigated by monitoring both the emission current-voltage curve and emission site density. A very low turn-on field of electron emission was found to be 0.5 V/mum for the hydrogenated surface and 4 V/mum for the hydrogen free surface. The emission site densities were significantly reduced from 3 x 10(3) to 2 x 10(2) cm(-2) at 5 V/mum after the surface hydrogen desorption. The low turn-on field and high emission site density on the hydrogenated surface are explained by the low work function of diamond combined with graphitic patches on the surface.