Much of the electronics used to support power systems and enable safety systems resides underhood where operating temperatures are much higher than in traditional consumer applications. Underhood electronics may be subjected to sustained high temperature environment 150°C for long period of time during operation. However, there is insufficient information about the viscoelasticity of epoxy molding compound stored in sustained high temperature for long period of time. In this paper, two different types of epoxy molding compounds have been prepared and aged under two different temperatures: 100°C and 150°C. Multi-frequency scan dynamic mechanical analyzer (DMA) test has been conducted to study the viscoelasticity evolution from pristine, 40 days, 80 days, 120 days. The master curve has been obtained and the prony parameters of EMCs have been calculated. The aging effect of linear viscoelasticity has been discussed.