PURPOSE: Glioblastoma (GBM) exhibits a high ROS character, giving rise to an immunosuppressive microenvironment and tumor vascular abnormality. This study investigated the potential effect of N-acetylcysteine (NAC), an antioxidant, on primary and recurrent mouse brain tumors. METHODS: We measured reactive oxygen species (ROS)/ glutathione (GSH) levels in human GBM. Additionally, we conducted NAC trials on primary mouse brain tumor models (GL261-Luc, CT2A-Luc) and a recurrent mouse GBM model (GL261-iCasp9-Luc). After brain tumor inoculation, mice received a daily 100 mg/kg NAC treatment, and the tumor volume was monitored via IVIS imaging. The efficacy of NAC was evaluated through survival time, tumor volume, ROS/GSH levels, M1/M2 macrophages, immune cells infiltration, and tumor vascularization. RESULTS: Human GBM suffered from significant oxidative stress. With NAC treatment, mouse brain tumors exhibited a lower ROS level, more M1-like tumor-associated macrophages/microglia (TAMs), more CD8 + T cell infiltration, and a normalized vascular character. NAC inhibited tumor growth and suppressed recurrence in mouse brain tumor models. CONCLUSION: NAC is a promising adjunctive drug to remodel the brain tumors microenvironment.