The non-isothermal crystallization behaviors of poly (trimethylene terephthalate) (PTT), poly (trimethylene terephthalate-co-isophthalate) (PTTI) and poly (trimethylene terephthalate isophthalate-co-polyethylene glycol) (PTTI-PEG) were investigated using Differential Scanning Calorimetry (DSC). The experimental results showed the crystallization temperature of PTT, PTTI and PTTI-PEG increased when the heating rate increased, and the Avrami exponents n of PTT, PTTI ranged from 3.5 to 5.5, and it was assumed that the non-isothermal crystallization mechanism for PTT and PTTI was the combination of homogenous and heterogeneous nucleation. However, the n value of PTTI-PEG was below 2.5, and the non-isothermal crystallization mechanism was ascribed to heterogeneous nucleation different from that of PTT and PTTI. The activation energy of PTTI increased with the IPA ratios increasing, and the activation energy of PTTI-PEG was the highest, suggesting that the crystallization rate was more sensitive to the temperature in comparison with PTT. Therefore, it was of great importance to control the temperature in processing.