In this study, the adhesion strength of three epoxy resins, which are used as basic materials for epoxy molding compound (EMC) in microelectronics, to copper leadframe was determined using the peel test. The epoxy resins used were O-cresol Novolac (OCN), dicyclopentadiene (DCPD), and biphenyl sulfide (BIPHS) epoxy resins. It was found that DCPD showed the highest peel strength and OCN had the lowest value. The difference in the peel strength was explained by investigating the physical and mechanical properties, as well as the surface properties of the epoxy resins. These properties included the surface energy, viscosity and gelation time, fracture toughness, and the coefficient of thermal expansion. As a result of the lower viscosity of BIPHS and DCPD than OCN epoxy resin, BIPHS and DCPD have a better peel strength than OCN. The DCPD resin has a better peel strength than BIPHS because of its higher fracture toughness.