Effect of IPA (isophthalic acid) content on the thermomechanical properties of a copolyester was studied by means of DSC (differential scanning calorimeter) and TMA (thermal mechanical analyzer: static force mode of dynamic mechanical analyzer). Copolyester including IPA has been used as a binder for polyester and nylon nonwovens. It was found that melting peaks of 30 mol% IPA added copolyester existed in TMA curves, but did not appear in DSC thermograms. The melting temperature rapidly decreased with the addition of 0 to 30 mol% IPA. Thermal mechanical studies clearly showed that when more than 35 mol% IPA was added the copolyester was a completely amorphous polymer. Thermal properties measured by DSC and dynamic viscosity behaviour showed similar behaviour with the amount of addition of IPA. The dynamic viscosity of copolyester in which from 0 to 20 mol% IPA was added had two linear temperature dependences with the slope changing at the solidification point. Above 30 mol%, linear temperature dependence with no change in the slope at the solidification point was observed.