A chemical analysis method was developed to evaluate the degree of chemical degradation for radial shaft seals after driving in a market. The obtained samples were analyzed with Fourier transform infrared spectroscopy (FT-IR) and surface hardness measurement. As the driving mileage increased, polymer structure changed and surface hardness increased. The degradation of the lip of radial shaft seals was reproduced in a similar environment in our laboratory, and we determined that the factors responsible for degradation are oil additives, temperature, and air. Moreover, the relationship between various types of FKM polymer and their resistance against oil additives was obtained using the molecular orbital simulation method.