A combination of extensional rheological and in situ synchrotron radiation small-angel X-ray scattering (SR-SAXS) measurements was introduced to investigate critical strain epsilon* for shish formation and validate whether coil-stretch transition or stretched-network is responsible for shish-kebab formation in high density polyethylene melt. With strain rates (epsilon) over dot larger than a specific value, the critical strain epsilon* required to induce shish formation turns Out to be a constant of about 1.57, which ensure full extension of chain segments with critical entanglement molecular weight locked between two adjacent entanglement points. The results clearly demonstrate that the formation of shish-kebab in polymer melt steins from stretched network instead of coil-stretch transition.