Effects of high valence ions V 4+ and Ni 3+ substitution on superconductivity are studied systematically for the electron-doped superconducting Pr 1.85 Ce 0.15 CuO 4−δ (PCCO) system. The results indicate that very small amount of V 4+ substitution (x⩽0.04) for Cu 2+ site improves the superconductivity, but further substitution (x⩾0.06) will depress the superconductivity. Contrasting with V 4+ , Ni 3+ substitution shows a strong suppression on superconductivity, even for extremely small amount substitution of 0.01. Both the change of lattice parameters and the similar 3d electron structure of V 4+ ion to the Cu 2+ ion support the improvement of superconductivity while x⩽0.04 in the V-substituted PCCO system. The results of Hall effect indicate that high valence ion substitution for Cu 2+ increase the carrier concentration, which is also one reason for the improvement of superconductivity of V 4+ substitution PCCO.