The poly(butylene succinate-co-terephthalate) (PBST) copolyesters with different monomer composition were synthesized by polycondensation. The isothermal crystallization kinetics of the prepared PBST has been investigated by differential scanning calorimeter (DSC).According to Avrami equation,the exponent n for all the copolyesters is independent of the crystallization temperature and content of BT units, and is close to 3, which probably corresponds to the heterogeneous nucleation and a 3-dimensional spherulitic growth. Polarized light micrographs confirmed the result.