The temperature of A.S.T.M. type warmth retaining tester is regulated by on-off type control method. The author developed the phase control type warmth retaining tester. This study is done to know whether the phase control warmth retaining tester can be replaced by on-off control method. Ten specimens of cotton fabric were tested with these two kinds of testers. The data of experiment were statistically analized and the following conclusions were obtained. 1. In the value of heat transmittance by on-off type apparatus, there was no clear evidence for differences of mean among 10 specimens, but the data of experiment by phase control type apparatus made significant differences among 10 specimens. 2. There was significant difference of variance between two types of warmth retaining tester.