The properties of carbon fibers made fiom PAN are controlled by the heat treatment conditions. The length changes during heat treatment of high strenffh homo-PAN and co-PAN(acrylonitrile/acrylamide : 9812wt%) fibers in nitrogen atmosphere were measured by monitoring the elongation-shrinkage behavior under variable loading from 0.02 to 0.1g/d. A few structure parameters such as aromaticity index and crystalline orientation etc. were determined for the fibers heat-treated by linear heating to 27$0^{circ}C$. The onset temperature of the chemical shrinkage during heat treatment coincided with the one of the exothermic peak of DSC thermoyams. When PAN fibers are heated under variable loads, the fiber length changes by various ways, namely physcial shrinkage, balance-elongation and chemical shrinkage. The co-PAN fibers tend to shrink more readily than homo-PAN fibers in that the onset of chemical shrinkage of co-PAN is shified to lower temperature. The total shrinkage of PAN fibers decreased with increasing load.