The reactor pressure vessel (RPV) is a unique and irreplaceable Class A component in the nuclear power plants (NPPs). Therefore, it directly determines the operational lifetime of NPPs. However, RPV steel is subjected to high-energy neutron radiation during its in-service lifetime, which can result in embrittlement and hardening of the RPV steel. The irradiation can induced high-density MnNi-rich clusters in irradiated RPV steels. Once nucleated, these MnNi-rich clusters can rapidly grow to large volume fraction, which causes the sudden and severe degradation of mechanical properties of the RPV steel. In this study, the RPV steel and model alloys subjected to aging treatment and high-energy Fe ion irradiation are used as experimental materials to study the formation mechanism of MnNi-rich clusters and effect of dislocations on vacancy-like defects and MnNi-rich clusters. The experimental results of the aged RPV steel showed that high-density B2 MnNi phases were found in aged RPV steel. The precipitation of these MnNi-rich ordered phases has been mainly dominated by the thermodynamic of RPV-steel system. Moreover, high-density dislocations can facilitate the nucleation and growth of the B2 ordered phases. Meanwhile, the strengthening mechanism of these ordered phases is dislocation shearing mechanism. The strengthening behavior of these ordered phases is mainly dominated by the coherency strengthening and ordered strengthening in dislocation shearing mechanism. Some nanoscale NiSi-rich sub-precipitates were also found within the cementite of aged RPV steel. These NiSi-rich sub-precipitates have the same crystalline structure as cementite, i.e., an orthorhombic lattice with the space group Pnma (no. 62). The formation of these sub-precipitates facilitates to decrease the energy of the cementite and improve its stability.The study of the irradiated samples with different dislocation densities showed that high-dislocation dislocations caused the decline of the S parameter and the increasing of the W parameter. However, these changes for FeCu alloy is mainly ascribed to the formation of Cu-vacancy complexes and sink effect of dislocations on point defects. For the RPV sample, only sink effect of dislocations on point defects could suppress the formation and growth of large vacancy-like defects, causing the decline of S parameter. The result of MD simulations indicated that screw dislocations can facilitate annihilation between interstitial- and vacancy-like defects, thus reducing the overall mean size and density of vacancy-like clusters.The experimental result of the irradiated FeMnNi alloy with different dislocation densities showed that dense dislocations can reduce the response of irradiation-induced hardening. According to results of PAS and APT, dense dislocations can suppress the formation of large vacancy-type defects and MnNi-rich clusters. By analyzing the effect of dislocations on PDs, diffusion of Mn and Ni atoms, and nucleation barrier of MnNi-rich clusters, we found that the formation of MnNi-rich clusters is mainly dominated by the radiation-induced segregation mechanism. However, high-density dislocations can facilitate to annihilate irradiation-induced PDs and cause the decline of defect-solute coupling fluxes, which are responsible for a reduction in number density and mole fraction of MnNi-rich clusters.