Field emission characteristics of heavily phosphorus (P)-doped homoepitaxial diamond is measured. Temperature dependent resistivity measurements indicate that hopping dominant transport in heavily P-doped homoepitaxial diamond. Field emission current versus anode voltage characteristics divide into three distinct regions. The effective electron affinity of O-terminated diamond surface is calculated to be 1.5 eV from F–N plot. This value is in reasonable agreement with data evaluated from total photo-electron yield spectroscopy measurements. Using these data, we introduce field emission process of O-terminated heavily P-doped diamond: i) emission of electrons from the conduction band minimum, ii) depletion of electrons in conduction band and iii) field emission from the phosphorus doping level.