Enhancement of bone substitute's biocompatibility may accelerate healing of surrounding bone. Although widely used as a biodegradable alloplastic bone substitute for alveolar bone augmentation, the osteocompatibility of beta-tricalcium phosphate (β-TCP) remains to be proven. The adverse cellular response to biomaterials is associated with oxidative stress. We hypothesized that commercially available β-TCP granules for clinical use, caused oxidative stress and was not optimal in osteocompatibility and that application of antioxidant amino acid derivative N-acetyl cysteine (NAC) would improve osteoblastic responses to the material. Only 20% of rat calvarial osteoblasts cultured on β-TCP granules remained viable at 24 h after seeding as opposed to 90% on polystyrene. Cell death on β-TCP granules was characterized by necrosis. However, the percentage of viable osteoblasts cultured on β-TCP granules showed a 100% increase with pre-treatment with NAC. NAC restored suppressed alkaline phosphatase activity on β-TCP granules at day 5. Intracellular ROS level on β-TCP granules was 16-fold greater than that on polystyrene, but decreased by half with pre-treatment with NAC. Cell death and intracellular ROS elevation were also induced in polystyrene culture under β-TCP granules even when the osteoblasts were not in direct contact with the β-TCP granules. NAC, however, prevented induction of cell death and elevation of intracellular ROS under β-TCP granules. These results indicate that commercially available β-TCP granules negatively affect cultured osteoblastic viability and function via oxidative stress and that NAC improves these negative responses to the material. This implies enhanced bone regeneration around biodegradable calcium phosphate-based bone substitute by NAC.