The goal of this publication is to investigate and understand the mechanism of the deterioration effect in radiation embrittlement of reactor pressure vessel steels of Ni–Cr–Mo–V or Mn–Ni–Cr–Mo types with a high nickel content (>1.5 wt%) in nuclear power plants. Eleven institutes from eight countries and the European Union participated in the Coordinated Research Project (CRP) entitled Mechanism of Nickel Effect in Radiation Embrittlement of RPV Material, and six institutes conducted the irradiation experiments with the CRP materials. In addition to the irradiation and testing of those materials, irradiation experiments with various national steels were conducted. This publication provides the results of all the data obtained by the participants. It is generally accepted that the presence of nickel in RPV steels increases their sensitivity to neutron induced embrittlement even at low phosphorus and copper concentrations.