Poly(trimethylene terephthalate-co-trimethylene isophthalate) [P(TT-co-TI)] copolymers with various contents of TI unit were synthesized via a two-step process of transesterification and polycondensation in a high pressure reactor in order to modify the thermal properties of PTT. Effects of TI content on the crystallization and other thermal properties were investigated by DSC, TGA, polarizing microscope, and synchrotron WAXD analysis. DSC analysis indicated that the cold crystallization temperature ($T_{cc}$) increased and glass transition temperature ($T_g$) and melting temperature ($T_m$) decreased with increasing TI content in the copolymer. The radial growth rate of PTT spherulite formed during the decrease of melt-crystallization as TI content increases. The growth and melting of the PTT crystallite were also analyzed in-situ by synchrotron WAXD patterns. A decrease in both $T_m$ during the heating process and melt-crystallization temperature ($T_{mc}$) during the cooling process for P(TT-co-TI)s were confirmed by WAXD patterns. Although the incorporation of the non-crystallizing comonomer unit of TI into the crystalline body of PTT reduced the crystallization rate of PTT, it did not affect the spherulite formation and crystal structure of PTT. Isothermal TGA analysis revealed that the activation energy for isothermal degradation with weight loss up to 10% of P(TT-co-TI)s was 200~240 kJ/mol, depending on the weight loss and TI content in the copolymer.