The nanoscale Cu-rich precipitates in reactor pressure vessel (RPV) model steel after water quenching, tempering and thermal aging at 400 ℃ has been investigated by extraction replica (ER) and high resolution transmission electron microscopy (HRTEM). Samples were coldrolled by 20%~30% after aging to study the deformation characterization of Cu-rich precipitates. The results show that the deformation characterization of the nanometer Cu-rich precipitates is complicated due to the restriction of αFe matrix. Different deformation mechanisms are detected. When the Cu-rich precipitates are located in a favorable orientation, they will be deformed by slipping together with the deformation of α-Fe matrix to show the particles of “soft” characteristic. When the Cu-rich precipitates are located in an unfavorable orientation, they will be deformed by twinning to form the spokelike twins within a Curich precipitate, even a martensitic phase transformation induced by strain. In this case, the Curich precipitates show the particles of “hard” characteristic. Therefore the precipitation hardening occurs after the formation of Cu-rich precipitates in α-Fe matrix.