It was found that incorporation of small amount of some rigid arylate-units, such as 4,4`-biphenylene terephthalate or paraphenylene terephthalate, into poly(ethylene terephthalate) increased the crystallization rates of the resultant copolymers. When the comonomer content increased, increase in the nucleation density was observed, and it was accompanied with reduction of the induction time and increase in the crystallization rate. The copolymers containing around 10mol% of the comonomers showed the maximum crystallization rate of 2-3 times as fast as that of PET. On the other hand, incorporation of rather flexible and bulky diols instead of the rigid diols, such as bisphenol-A and bisphenol-S, did not induce such a remarkable crystallization behavior and only decreased the crystallizability compared to the homopolymer.