Abstract: This study investigated how the initial C-center nitrogen content, electron irradiation, vacuum annealing, and high pressure high temperature pretreatment affects NV center formation and charge-state regulation in HPHT type-Ib diamond. Two groups of HPHT diamond single crystals with different C-center nitrogen contents were used. The samples were treated by electron irradiation, vacuum annealing at 600 to 1000 ℃, and HPHT pretreatment at 5 GPa and 1100 to 1900 ℃. FTIR, Raman, and PL spectra were used to analyze nitrogen aggregation, lattice state, and NV-related photoluminescence. The results show that the high-nitrogen sample forms stronger NV-related emission, whereas irradiation-induced vacancies in the low-nitrogen sample tend to remain as GR1 centers. Increasing the vacuum annealing temperature enhances the total NV-related emission but reduces the relative response of negatively charged NV centers. HPHT pretreatment does not directly produce many NV centers. Instead, it promotes nitrogen aggregation and modifies the apparent charge-state ratio after subsequent irradiation and annealing. These results indicate that HPHT pretreatment can serve as a preceding processing parameter for tuning the initial defect state before irradiation and regulating the later charge-state response of NV centers.