No unified and definitive theory exists regarding the formation mechanism of MnNiSi clusters, a type of irradiation damage defect in reactor pressure vessel (RPV) steel. However, experiments have shown that Ni has a substantial impact on the nucleation of these clusters. Thus, to investigate the role of Ni clusters in the formation mechanism of MnNiSi clusters in RPV steels, this study employed first-principles simulations using VASP software. The simulations calculated the capture ability of close-packed Ni₃ and planar Ni₄ clusters, acting as nucleation cores, toward solute atoms, vacancies, or dumbbells. The nucleation pathways following their combination were also calculated. The results indicated that both Ni₃ and Ni₄ clusters could serve as nucleation cores for MnNiSi clusters. The Ni₃ cluster could stably bind with a Si atom or a vacancy, whereas the Ni₄ cluster could stably bind with a Mn atom, a Si atom, or a vacancy. Specifically, when considering only solute atom capture, the nucleation pathway of the Ni₃ cluster could grow into part of the γ₂ phase structure, whereas that of the Ni₄ cluster could grow into part of the G phase structure. When incorporating dumbbell and vacancy interactions, the Ni₄ cluster could nucleate and grow into multiple distinct phase structures, including the G phase.