This study investigated the structures and properties of cationic dyeable poly(ethylene terephthalate) (CDP) and easy cationic dyeable poly(ethylene terephthalate) (ECDP) by FTIR, X-ray diffraction, mechanical test, and DSC. The results showed that the FTIR spectrums of CDP and ester-type ECDP fibers were similar to the that of PET. It suggested that the adding of the third monomer and the fourth monomer had good compatible with the original structure unit of PET. the position of X-ray diffraction peak of CDP and ester-type ECDP were the same as which of PET, the third monomer and the fourth monomer did not pack into the original polyester unit cell but concentrated at the polyester crystalline surface and the amorphous regions.With adding the third monmome and the fourth monmomer, the initial modulus of the CDP and ester-type ECDP fibers became inferior. But the breaking elongation of them increased. The DSC suggested that The melting range of CDP and ECDP finished fiber were wider than that of PET fiber, but the melting point and crystallinity were less than them of PET fiber.