The process of open-porous structure development in high density polyethylene (HDPE) films during uniaxial deformation in supercritical carbon dioxide (SC-CO2) fluid at 35 degrees C and 10 MPa has been studied and visualized by means of atomic force microscopy. We suggest that the supercritical fluid act as adsorption-active medium, and the porous structure is developed via the crazing mechanism due to the increasing the distance between of lamellae and the formation of oriented separate fibrils in the intercrystallite space. Effective bulk porosity of the films has been up to 40%. Small-angle X-ray scattering studies and ethanol permeability measurements have revealed that the pores and fibrils are about 10 nm in diameter. The prepared nanoporous materials exhibit good vapor permeability. Structural and mechanical behavior of the prepared porous films has been investigated. Large reversible deformation (up to 80%) of HDPE in the SC-CO2 has been observed. Repeated drawing of the shrunk films in air under ambient conditions has led to the open-porous structure recovery.