Aiming to reveal the effects of mechanical energy and thermal effect on micropore nucleation and growth in oriented high density polyethylene (HDPE) film, stress relaxation after cold stretching was imposed at low and elevated temperature with strain holding constant, respectively, and corresponding structure evolution was tracked by in situ and ex situ small angle X-ray scattering (SAXS). It was found that stress-induced density fluctuation during cold stretching could be completely recovered as soon as the stress was unloaded, which was called as micropore embryo. Mechanical energy release at low temperature can promote micropore nucleation after an induction period. However, the growth of micropore nuclei is inhomogeneous, developing as non-uniform micropores with poor interconnectivity during hot stretching. While during temperature elevation, micropore embryos can be converted into evenly distributed micropores together with formation of fibrils, which can supply growth sites for through pores during hot stretching. Consequently, microporous membranes with narrow micropore size distribution and good permeability could be obtained after the subsequent hot stretching. (C) 2017 Elsevier Ltd. All rights reserved.