At the last Congress of Bioclimatology in Sydney the new study group was established to cope with the problem of Universal Thermal Climate Index. It was assumed the new index should base on the human heat balance analysis. The index should be applicable in various climatic zones and in the wide range of meteorological conditions all over the world. The aim of the paper is to present the principles of the new index, called Subjective Temperature Index (STI). Human heat balance depends both, on external (meteorological) and internal (physiological) factors. However, some physiological parameters of subjects are under the influence of ambient conditions. In the human heat balance equation: M+Q+C+E+Res=S the M is metabolic heat production, Q - radiation balance of man, C - heat exchange by convection, E - heat loss by evaporation and Res - heat loss by respiration. S is net heat storage. The effect of man-environment heat exchange is subjectively felt as our response to the signals from heat and cool receptors distributed in the human skin. Skin temperature is an active component of the human heat balance. It depends on internal heat production and its transfer to skin surface, on ambient thermal stimuli and on the intensity of heat exchange. STI includes both, objective and subjective interactions between man and his thermal environment. In STI consideration ambient thermal conditions are represented by mean radiant temperature (Mrt). Its effect on subjective thermal sensations is modified by the resultant value of man-environment heat exchange (S*) which is formed under the influence of temporal cooling of skin by evaporation. At S*<0 STI=Mrt-[|S*|0.75/(5.39 10^-8 )+273⁴]^-.25}-273 and at S*>=0 STI=Mrt+[|S*|0.75/(5.39 10^-8)+273⁴]^0.25}-273. Applicability of Subjective Temperature Index was verified for arctic climate (temperature of -50°C, wind speed up to 32 m/s), temperate clime and for hot dry and hot humid climates (temperature up to 50°C, vapour pressure range 10-60 hPa) both, for absence and for presence of solar radiation. The results of simulations shows great concordance between subjective thermal sensations and actual meteorological conditions.