The effect of heat treatment and mechanical stress on the dielectic strength of epoxy resin film is studied. The film with 30-210 .mu.m thickness are casted at room temperature for 8 hours and post cured at the range of temperature 60-180 .deg. C, and the heat setting time are two kind, ie, 2 and 10 hours. The samples are made with Stycast 1266 that the primary compound is diglycidyl ether of bisphenol A, and the hardner is denaturated polyamines. Under no mechanical stresses, the maximum dielectric strength of the sample is obtained for the sample heat treated for 2 hours at 150 .deg. C. However, the best dielectric strength characteristics under compressive stress is obtained for the sample heat treated at 90 .deg. C and 120 .deg. C. The dielectric strength of the sample are also affected significantly by the cooling velocity of the sample after post heat setting at given temperature.