The results of determination of thermal stability of mineral and partly synthetic motor oils are presented. The criterion of thermal stability is substantiated and the correlation between it and antiwear properties of the oils is determined. Direct photometric measurement method, which helps obtain an integral parameter of accumulation of all degradation products in the studied oil that can be determined from the luminous flux absorption coefficient, is chosen as the basic method of evaluation of thermal degradation processes. The essential feature of the method consists in comprehensive evaluation of thermal stability and antiwear properties of lube oils from the parameters, namely, the coefficient of luminous flux absorption, relative viscosity, vaporizability, and wear. Correlations of optical properties, viscosity, vaporizability, and antiwear properties of the lube oils with thermostating temperature are obtained, and based on them the limiting temperature of lube oil performing ability was established.