Copolyesters were synthesized by using ring-opening polymerization of cyclic poly(butylene terephthalate) oligomers (c-PBT) and E-caprolactone monomer (CL) in the presence ofstannoxane catalyst at an elevated temperature. The in-situ copolymerization was conducted with various c-PBT/CL feed ratios. The copolyesters were characterized by GPC, NMR, and FT-IR techniques. Phase separation and the relaxation behavior of the resultant copolyesters were investigated by DSC, DMTA, and dielectric spectroscopy measurements. Mechanical properties (stress-strain behavior) are also reported and can be adjusted by varying the E-caprolactone content of the copolymer.