The crystal structure and isothermal crystallization behavior of random copolyesters based on ethylene terephthalate(ET) and caprolactone(CL) units were investigated by melting point depression, X-ray diffraction and polarizing microscope equipped with a hot stage. The melting point depression begavior of P(ET/CL) random copolyesters with increasing contents of CL units was closest to that predicted by the Baur equation rather than the Flory equation. Therefore P(ET/CL) random copolyesters appear to exhibit pseudoeutectic type crystal structure where the melting point depression depends not only on the composition but also the segment lengths. In the X-ray analysis, the diffraction angle and d-spacing values of P(ET/CL) random copolyesters were changed. It appears that the unit cell structure has been deformed slightly. Thus small amounts of CL unit appear to have been incorporated into the crystal as defects. The crystallization rate of the P(ET/CL) random cololymers depended on the segment length and composition of ET units, and incorporation of CL units generally decreased the crystallization rate of P(ET/CL) copolymers. However, the copolymer containing 13 mole % CL units exhibited an increase in the crystallization rate. This appears to be due to the enhanced mobility of the copolymer chain and the sufficiently long ET segments which insures anchoring of the segments to the growing crystal.