Purpose: To investigate the effects of irradiation on the phenotypic expression of the MC3T3-El osteoblastic cell line, particularly an the expression of type I collagen and alkaline phosphatase mRNA. Materials and Methods: Cells were irradiated with a single dose of 0.5, 1, 2, 4, and 8 Gy at a dose rate of 5.38 Gy/min using a cesium 137 irradiator. The specimens were then harvested and RNA extraction was carried out at 1 and 3 days after irradiation. The extracted RNA strands were reverse-transcribed and the resulting cDNA fragments were amplified by PCR. Results: The irradiated cells demonstrated a dose-dependent increase in type I collagen mRNA expression relative to the control group, with a maximum level of type I collagen mRNA expression occurring at 8 Gy. The degree of type I collagen mRNA expression increased significantly at 1 day after irradiation, but little differences were found between the control group and at the 3rd day. The amount of alkaline phosphatase mRNA expression increased significantly at land 3 days after irradiation in the 1 Gy exposed group compared with the control group. Conclusion: The amount of type I collagen and alkaline phosphatase mRNA expression increased significantly 1 day after irradiation when compared with the control group.