An analysis of the flow of polymer melts as well as of melt fracture at high shear rates has been carried out. On the basis of the flow properties of polyethylene, polypropylene, polystyrene, polymethylmetacrylate melts, and silicone rubbers measured over a wide range of shear rates from 10−3up to 103sec−1, the following conclusions have been drawn: Under low rates of shear the behavior of polymer melts is of Newtonian nature, whereas at high rates the character of flow depends on the value of the dimensionless criterion&kgr;=&tgr;/&eegr;N&ggr;˙=&kgr;3&tgr;·R/&eegr;N·V, where &tgr; is shear stress&ggr;˙is shear rate, &eegr;Nis Newtonian viscosity,Ris characteristic dimension, andVis mean velocity of flow. The fracture of polymer melts occurs above a critical constant value of the number &kgr;: &kgr;crit=0.025. Available flow data, analysed by the method of reduced variables, show that most previously suggested criteria are essentially equivalent. This treatment eliminates their restricted validity and technological applicability and brings the description of fracture close to molecular theories of non‐Newtonian flow.