A series of copolyesters based on different ratios of polyethylene terephthalate (PET) to polyethylene isophthalate (PEI) have been synthesized. With the involvement of PEI, the copolyesters become less crystallizable and even amorphous when PEI content is above 20%. The WAXD profiles of the crystallizable copolyesters infer that the crystals come from PET homopolymer. DSC cooling runs indicated that the copolyesters with PEI no more than 15% are easily crystallizable, while the copolyester with 20% PEI is not easily crystallized at a cooling rate above 5 degrees C/min. Heating runs indicated that the copolyesters with PEI below 20% show melting processes. Nonuniform results were provided by WAXD and DSC, however, the effect of PEI on the ability of crystallization was deduced similarly from WAXD and DSC. Glass transition temperatures have been measured by DSC. Due to the flexibility of PEI chain, glass transition temperatures of the copolyesters decrease linearly with increasing composition of PEI as predicted by the principle of additive contribution.