A series of poly(ethylene terephthalate/polycaprolactone) (PET/PCL) copolyesters were prepared by adding PCL with a weight-average molecular weight of 40,000 to the PET oligomer during polycondensation, and the composition and physical properties of the copolymers were investigated. In the case of copolyesters containing caprolactone (CL) units less than 26 mol %, the melting and crystallization temperatures were decreased with increasing CL content. They exhibited wide-angle X-ray diffraction (WAXD) patterns similar to PET. However, the variation of d-spacing on the WAXD pattern suggested that a small amount of CL could be included into the crystal of ethylene (ET) because of strong hydrogen bonding between crystallizable ET blocks. In the copolyesters containing CL components higher than 38 mol %, they exhibited thermal behavior and WAXD patterns typical of amorphous random copolymers. The copolyester composition has a decisive effect on the rheological properties. The copolyesters with CL contents higher than 38 mol % exhibited a viscosity behavior similar to Bingham fluid, although the copolyesters with CL contents lower than 26 mol % exhibited a Newtonian flow behavior similar to PET. In addition, the value of the loss tangent of the copolyester melts notably decreased with an increasing CL content, and the bulk exhibited rubberlike behavior.