The present paper is Part 1 of two consecutive studies. Part 1 describes three subjects: definition of the equivalent temperature (teq), measurements of teq using a clothed thermal manikin in a vehicle cabin, and modeling of the clothed thermal manikin for teq simulation. After defining teq, a method for measuring teq with a clothed thermal manikin was examined. Two techniques were proposed in this study: the definition of “the total heat transfer coefficient between the skin surface and the environment in a standard environment (hcal)” based on the thermal insulation of clothing (Icl), and a method of measuring Icl in consideration of the area factor (fcl), which indicates the ratio of the clothing surface to the manikin surface area. Then, teq was measured in an actual vehicle cabin by the proposed method under two conditions: a summer cooling condition with solar radiation and a winter heating condition without solar radiation. The results showed that teq, including the effects of the air temperature, air velocity and thermal radiation, was measured properly. Subsequently, a numerical thermal manikin was developed. The geometry of the clothed manikin was modeled by 3D laser scanning. The procedure for evaluating teq using the numerical thermal manikin was defined, based on a heat exchange model in consideration of the fcl effect. The evaluation procedure can be applied to all manikin control modes described in ISO 14505-2, namely, a constant temperature mode, a constant heat flux mode and a comfort equation mode. Finally, the calculation accuracy obtained with the numerical thermal manikin was confirmed under a uniform environment. The results showed good agreement with the measured data. The numerical thermal manikin was used in a vehicle simulation in Part 2, and the simulation results were validated in comparison with the vehicle measurements made in the first part.