was solution-polymerized in dimethyl sulfoxide(DMSO) and tertiary butyl alcohol(TBA) at 30, 40, 50℃ using a low temperature initiator, 2,2'-azobis(2,4-dimethylvaleronitrile) (ADMVN). The low temperature polymerization using ADMVN, DMSO, and TBA is to be successful in obtaining high molecular weight polyacrylonitrile(PAN) with less branches by solution polymerization. Through a polymerization of AN in DMSO at 30℃, PAN having viscosity-average molecular weight(Mv) of 931,000 was obtained. And then, during AN solution polymerization in DMSO and TBA using a cosolvent system, the in-situ formation of microfibrillar structure has been discovered at the cosolvent composition of 24/1(VDMSO/ VTBA). The simultaneous process of gelation and phase separation of long chain molecules may explain the in-situ formation of PAN fibers during polymerization.