25-Hydroxycholesterol (cholest-5-ene-3, 25-diol, 25-OHC) showed the cytotoxicity on HeLa human cervix and NCI-H460 human lung cancer cells, $0.5{mu}M;of;50\%$ inhibitory concentration. We studied 25-OHC as the possibility of radiation sensitizer. The combination effect of 25-OHC and y-irradiation measured using flow cytometer with propidium iodide stained cells. The combined treatment of 25-OHC and $gamma-irradiation$ did not show significant enhancing effects on Hela and NCI-H460 cells.