A new gate insulating film consisting of P3N3 was formed on an InP surface by a new CVD method. In this method a mixture of PH3 and NH3 was used as a reagent gas. The film showed ohmic characteristics with 3×1015 Ωcm at electric fields up to 4×106 V/cm. The surface state density at the P3N3–InP interface decreased with decreasing deposition temperature, TD, and it was reduced to 2×1012/cm2·eV near the conduction band edge when TD=500°C. Below this temperature, the film became non-stoichiometric (P rich) and exhibited hygroscopic deterioration with water vapor. An inversion-mode n-channel InP-MISFET was fabricated using P3N3 film as a gate insulator. An effective electron mobility of 1000∼1640 cm2/V·sec was obtained for this device.