After considerably long time of transesterification reactions between poly(ethylene terephthalate) (PET) and bisphenol-A polycarbonate (PC) in the molten state, random copolymers, referred to be TCET's, can be obtained, which have fairly good compatibilizing effect on the immiscible PC/PET blend. The compatibilizing effect of these transesterification random copolymers is proved to be closely related to their compatibility with PET and PC. Being completely compatible both with PET and PC, the TCET50 copolymer with 50 wt.-% ethylene terephthalate content is an efficient compatibilizer, it can greatly improve the compatibility between PET and PC. With increasing content of the TCET50 copolymer in the PC/PET/TCET50 ternary blend, the two glass transition temperatures, which belong to the PET-rich and PC-rich phase respectively, approach each other gradually. When the content of the TCET50 copolymer in the blend reaches 60 wt.-%, only one glass transition temperature can be detected by differential scanning calorimetry (DSC). The TCET30 and TCET70 copolymer, which have 30 and 70 wt.-% ethylene terephthalate content respectively, are less efficient in compatibilizing the PC/PET blend, since the TCET30 copolymer and PET, as well as the TCET70 copolymer and PC, are compatible to a certain degree instead of being completely compatible.