The copolyesters of 1,4-butanediol, terephthalic acid, isophthalic acid, and adipic acid with different compositions were prepared by melt polycondensation. The intrinsic viscosity of the copolyesters was controlled to be about 0.7-1.0 dL/g. Their second-run thermal properties were determined by DSC. The composition was found to be a major factor in the determination of the thermal transitions. The effect of the composition on the glass transition temperature and melt transition temperature was interesting. The glass transition temperature was found to be dependent mainly on the content of adipic acid. As the content of adipic acid increased, the glass transition decreased. When the content of terephthalic acid was not less than 35 mol % based on the total diacid monomers, the melting temperature was dependent mainly on the content of terephthalic acid and increased with increase of the content of terephthalic acid. When the content of terephthalic acid was less than 35 mol %, the melt temperature of the copolyesters did not follow a simple rule. However, the effect of the composition on their melting enthalpy showed no simple trend.