The rapid development of high-power devices has driven the requirement for high-temperature stable epoxy molding compounds. In this work, a designed polymer blend system consisting of cyanate ester/epoxy copolymers modified by polyimide (CE/EP-PI) has been studied. Polyimide used in this study has shown excellent dispersity in the cyanate ester and epoxy copolymer network (CE/EP), exhibiting homogeneous phase with a denser polymer network structure. With this polymer blend structure, CE/EP-PI system was proved to have a glass transition temperature as high as similar to 270 degrees C, increased modulus, and largely enhanced fracture toughness up to 2.06 MPa m(1/2). CE/EP-PI resins showed outstanding long-term stability at high temperature with low mass loss and increased fracture toughness after aging at 200 degrees C. This work provides a novel insight into the development of molding compounds based on polymer blends system with excellent high-temperature properties. (c) 2018 Wiley Periodicals, Inc.