The effects of molecular weight of polycaprolactone(PLC) and the content of catalyst on the ester interchange reactions between poly(ethylene terphthalate)(PET) and PCL during melt blending were studied The ester interchange reactions between PET and PCL were characterized by $^{1}H$-NMR analysis and the P(ET/CL) copolymers were also characterized by DSC, SEM and X-ray analyses. The lower molecular weight of PCL and the higher level of catalyst content resulted in a higher degree of ester interchange reactions. It appears that the block characteristics, such as block length and randomness of P(ET/CL) copolymers from melt blending viaester interchange reactions, can be controlled by the molecular weight of PCL and the content of catalyst. These results may be applied to the other poly(aromatic/aliphatic) copymer systems prepared via melt blending. The melting and crystallization behavior, crystal structure and morphology of P(ET/CL) copolymers varied with the molecular weight of PCL and catalyst content due to different degrees of ester interchange reaction.