The biocompatibility of zirconia-alumina composite was evaluated with HOS osteoblast like cell models. A total of 18 zirconia-alumina composite disc (diameter: 19mm and thickness: 1.5mm) were prepared and divided into two groups. Half of the discs were sandblasted with 50µm alumina particles. Mean values of surface roughness (Ra) were 0.1 µm and 0.9-1.48 µm for smooth and sandblasted sample respectively. The cell attachment, proliferation, and differentiation on the specimen were evaluated by the Real-Time Polymerase Chain Reaction (RT-PCR), Methylthiazole Sulfate (MTS) analysis, Alkaline Phosphatase (ALP) activity, and Scanning Electron Microscopy (SEM). There was no significant difference in cellular response between two groups. The analysis of the RT-PCR showed that the amount of Cyclin D1(mRNA expression) was not statistically significant different between two groups after 24 hours as well, however markedly decreased in the smooth surface after 72 hours. This indicated that the rough one might have a more favorable cellular proliferation compared to smooth one in long term evaluation. Further study will be necessary.