Abstract: The R.K. Gupta’s extentional viscometer improved by the authors was used tomeasure the extentional flow properities of polyacrylonitrile (PAN).It showed that modifiedJeffery Model developed by us could describe both the shear flow and elongational flow forpolyacrylonitrile. In the modified-model,compared with Jeffery Model, we have got a newcharicteristic parameter which only effect elongational flow.