This paper presents a physical foam producing method of porous hollow fiber membranes from polymer melt and supercritical carbon dioxide by extrusion. In the process described in this work, porous membranes can be produced without the use of organic solvents or expensive thermic or mechanical after-treatment. The porous structure and the permeability were controllable by adjusting the amount of injected CO2 and the pressure- and temperature-profile in the extruder. The resultant hollow fiber membranes have a very high number of pores with a size on the order of 5-10 mu m, a narrow pore size distribution and a finely interconnected open cell structure. The N-2 gas permeability and H2O permeability were measured to estimate the open cell structure.