In order to calculate exactly and quantitatively the radiant heat exchange between the human body and indoor space in low temperature radiant heating system, it is pursued to determine the clothing surface temperature. In this paper, first of all, the clothed body's heat resistance in the literature which have great influence on radiant heat exchange was investigated, and pursued about the applicable formula. And, for the sake of making it easy to calculate the applicable formula, a diagram has been designed to make it easy to find the value of the clothing surface temperature and the clothing heat resistance. Thus, this study may be used as a basic resource for not only quantitatively radiant heat exchange of clothed body and unclothed body in low temperature radiant heating system, but also designing the thermal environment in a room.