Caprolactam was copolymerized with hexamethylenediammonium adipate (6.6) and ρ-xy1y1enediammonium adipate(PXD .6) to form two series of random copolyamides : copolyamide 6/6.6 and copolyamides 6/PXD.6. The thermal shrinkage behaviors of drawn films of copolyamides 6/6.6 anti 6/PXD.6 were examined by thermomechanical analyzer(TMA) The shriilkage behaviors of the copolyamides show two characteristic steps with temperature, which can be explained in terms of ,structural variations. The effects of structure and contents of comonomer units on thermal shrinkage behavior are explained in terms of amorphous and crystalline state.