Design : A pilot, comparative study was conducted. Background : Heat wraps using bentonite (HWb) are commonly used in thermotherapy. In the Okayama University Misasa Medical Center, heat wraps using mud (HWm) produced by mixing soil with boiling water have also been used. The subjective thermal effects of HWm on patients with osteoarthritis of the knee have been reported. However, the objective thermal effects of HWm have not been examined. As such, the physicochemical thermal effects of HWm were analyzed and compared with those of HMb. Methods : The thermal effects of HWm and HWb were investigated regarding heat radiation, conduction, capacity and moisture content. Heat radiation and conduction were measured by changes in temperature of the heat wraps, in addition heat conduction was also assessed by blood flow in body surfaces (N=7). Heat capacity and moisture content were measured with a Differential Scanning Calorimeter. Results: Heat radiation and heat conduction of HWm were significantly greater than those of HWb (p<0.001). The specific heat capacity of HWm was 2.0 J/g/°C, and was 1.7 J/g/°C for HWb. Moisture content of HWm and HWb were 0.6 mg/g and 0.3 mg/g, respectively. Conclusion : Heat radiation, conduction and capacity of HWm are superior to those of HWb. Moreover, the moisture content in HWm is greater than that of HWb. These results indicate that the thermal effects of HWm would have more be physicochemical usefulness.