Effects of hydrogenation on electrical properties of InP epilayers grown on GaAs substrates by 'photochemical vapor deposition' have been investigated. The electrical properties were characterized by current-voltage and capacitance-voltage measurements at room temperature. Deep level transient spectroscopy technique was used as an auxiliary method to detect the deep level traps. Hydrogen plasma exposure at 250 °C for 3 h increased the reverse breakdown voltage by a factor of about 1.7 and the Schottky barrier height by 0.05 eV. In addition, the shallow and deep levels were substantially passivated. These results indicate that hydrogenation by the photochemical vapor deposition system for InP on GaAs is an excellent method to improve its device application.