An n-type phosphorous (P) doped epitaxial diamond film with high conductivity was grown on a C(001) substrate by gas source molecular beam epitaxy using methane and tri-n-butylphosphine. The electrical conductivity of the diamond film was measured to be 0.33 (Ω cm)-1 at 23 °C with its activation energy to be 0.12 eV. The Hall measurements showed n-type conduction and a carrier concentration of 1.6×1018 cm-3 at 400 °C, which is comparable to the P concentration determined by secondary-ion-mass spectroscopy. These indicate the formation of shallow P donors with high electrical activation efficiency. A p-n junction diode was fabricated by growing a P-doped epitaxial film on a boron-doped C(001) substrate, which showed for the first time a rectification ratio of ∼ 103 at 10 V.