Purpose : To investigate the effects of irradiation on transforming growth factor ${eta}_1$ (TGF-${eta}_1$) mRNA expression and calcific nodule formation in MC3T3-E1 osteoblastic cell line. Materials and Methods : Cells were cultured in alpha-minimum essential medium ($alpha$-MEM) supplemented with 10% fetal bovine serum and antibiotics. When the cells reached the level of 70-80% confluence, culture media were changed with $alpha$-MEM supplemented with 10% FBS, 5 mM $eta$-glycerol phosphate, and $50;{mu}g/mL$ ascorbic acid. Thereafter the cells were irradiated with a single dose of 2, 4, 6, 8 Gy at a dose rate of 1.5 Gy/min. The expression pattern of TGF-${eta}_1$ mRNA, calcium content and calcific nodule formation were examined on day 3, 7, 14, 21, 28, respectively, after the irradiation. Results : The amount of TGF-${eta}_1$ mRNA expression decreased significantly on day 7 after irradiation of 4, 6, 8 Gy. It also decreased on day 14 after irradiation of 6, 8 Gy. and decreased on day 21 after irradiation of 8 Gy. The amount of calcium deposition decreased significantly on day 7 after irradiation of 4, 8 Gy (P < 0.01) and showed a decreased tendency on day 14, 21 after irradiation of 4, 6, 8 Gy. The number of calcific nodules was decreased on day 7 after irradiation of 4, 8 Gy. Conclusion: Irradiation with a single dose of 4, 6, 8 Gy influences negatively the bone formation at the molecular level by affecting the TGF-${eta}_1$ mRNA expression that was associated with proliferation and the production of extracellular matrix in MC3T3-E1 osteoblastic cell line.