Field emission properties of phosphorus (P)-doped homoepitaxial diamonds are measured after hydrogen plasma treatment and wet chemical oxidization. We also measure field emission properties of H-terminated B-doped diamond for comparison. P-doped diamonds are grown on type Ib single crystalline diamond by microwave plasma chemical vapor deposition. The resistivity of the films is in the range of (5 - 8) X 10(2) Omega CM. Phosphorus concentration measured by secondary ion mass spectroscopy is in the range of (5 - 7) X 10(19) cm(-3). The threshold voltage for field emission of O-terminated P-doped diamond with positive electron affinity is lower than that of H-terminated P-doped diamond with negative electron affinity. Upward band bending due to ionized phosphorus donors exists at the H-terminated surface. This upward bend bending prevents electrons from reaching to the emitting surface. An effective emission barrier height of O-terminated P-doped diamond surface is estimated to be 1.3 eV using slopes of Fowler-Nordheim plots. This value is in good agreement with the reported electron affinities in literature. (c) 2006 American Vacuum Society.