Abstract: Non-Newtonan fluid is a kind of fluid whose components of stresstensor aren’t theliear funtions of compoents of the strain rate tensor.Non-Newtonianfluid is beingprocessed in many kinds of modern industry,Stability of flows for Non-Newtonianfluid is of important applicatuib,In this article we calculate subcritical thrdshold of flow which oecurs in polymer-processing when the melting substance is driven throughtwo parallel fixed boundaries.