Effect of thermal treatment in nitrogen or air on properties of polyacrylonitrile fibers of homopolymer as well as copolymer has been examined by means of TMA and DSC. The fibers thermally treated under load always showed an initial shrinkage and then an elongation followed by a late shrinkage. The copolymer fiber, however, showed only about 3% of the initial shrinkage while the homopolymer showed about 10%. An increase of the load applied on the fibers and the treatment temperature shortened the initial shrinkage as well as the late shrinkage. The copolymer fiber shrinker only about 5% unbar a load of 96.6mg/denier while about 25% under a load of 31.7mg/denier The homopolymer fiber showed no late shrinkage if treated in nitrogen but 22% of the shrinkage if treated in air, while the copolymer had little effect by the atmospheric gas on the degree of the late shrinkage. The density changed to over 1.3g/cm³ from 1.19g/cm³ if the fibers were treated for three hours at 260℃. The reaction in the fibers progressed fastest when the density of the fibers was ill the range of 1.3g/cm³ and 1.4g/cm³. It was noticed that the length of the fibers was fixed early while the diameter was shrinking.