This study is to establish a conversion model of thermal insulation ratio of the range 44.9~95.5% to clo(KS K 0466, ASTM D 1518), which enables to evaluate the necessary thermal insulating ability. In order to obtain a suitable regression equation of models to convert thermal insulation ratio to clo, coefficients of determination on each section and total data were searched in advance on the basis of thermal insulation ratio, the sections that coefficients of determination were high were adopted. As regression equations on adopted sections were derived, the optimum regression equations were decided after making a regression diagnosis. Also, reliability verification by new samples about the optimum regression equations was conducted. Results of this research make it easy to convert thermal insulation ratio(%) to clo during new development and design of military or commercial winter clothes and sleeping bag. Also, reduction of development costs and period can be expected.