The therapeutic effect of mammary breast cancer of rat (Sprague Dawley) was estimated by single and 5 fractionated irradiation of $Co^{60}-gamma-ray$. Response rates over 50a were 20, 43, 67, $80\%$ respectively by single dose irradiation of 800, 1,200, 1,600, 2,000 rad, and 20,38, 57, $88\%$ by 5 fractionated irradiation of 1,400, 2,100, 2,800, 3,500 rad. $50\%$ tumor control dose$(TCD_{50})$ were 1,282rad, 2,312rad respectively with single and fractionated irradiation.