Silicone rubber (Vinyl Methyl Silicone (VMQ)) had been widely applied to automotive parts such as engine seals and gaskets by its excellent resistance to high and low temperatures. Although silicone rubber for engine parts has been recently replaced by fluorocarbon rubber, which has superior high-temperature resistance, compatibility between silicone rubber and engine oil is still an important consideration in developing engine oils. For example, engine oil specifications, which are issued by International Lubricant Specification Advisory Committee (ILSAC) and Association des Constructeurs Europeens d'Automobiles (ACEA), require silicone rubber compatibility performance. However, there are still much unknown about the influences of each engine oil additive on silicone rubber. In this study, the influence of main additives in engine oils such as metal detergents and zinc dialkyl-dithiophopshates (ZDTPs) on silicone rubber and their mechanisms have been investigated. Specifically, silicone rubber test specimens were immersed in test oils containing such engine oil additives at 150°C for 2weeks, before physical properties of the test specimens were measured to compare influences of different types of additives. It was found that ZDTPs extremely reduce tensile strength of silicone rubber. Analyses of the test specimens by means of Fourier Transform Infrared Spectroscopy (FT-IR) revealed that the silicone rubbers were hydrolyzed to change in polymer structure. In addition, analyses of the test oils were also conducted in order to identify the species derived from the additives hydrolyzing silicone rubber. As the result of this experimental investigation, the mechanism of decomposition of silicone rubber by engine oil additives would be discussed.