In automobiles, Integrated Starter Generators (ISGs) are important components since they ensure significant fuel economy improvements. With motors that operate at high voltage such as ISGs, it is important to accurately know partial discharge inception voltages (PDIVs) for the assured insulation reliability of the motors.However, the PDIVs vary due to various factors including the environment (temperature, atmospheric pressure and humidity), materials (water absorption and degradation) and voltage waveforms. Consequently, it is not easy either empirically or analytically to ascertain the PDIVs in a complex environment (involving, for example, high temperature, low atmospheric pressure and high humidity) in which many factors vary simultaneously, as with invehicle environments. As a well-known method, PDIVs can be analyzed in terms of two voltage values, which are the breakdown voltage of the air (called “Paschen curve”) and the shared voltage of the air layer. However, this method is not sufficient for analyzing the PDIVs in an in-vehicle environment because it takes into account only the atmospheric pressure and the initial permittivity of materials from among many factors which affect PDIVs.The present paper analytically and empirically evaluates individual factors influencing PDIVs. Moreover, this paper presents the results of evaluation of sparsely reported complex environments and of analysis of their effects on PDIVs. Accurate elucidation of PDIVs in complex environments should contribute to the development of high-voltage motors that are smaller and more reliable than conventional motors.