Poly(4-methyl-1-pentene) hollow fibres, susceptible to physical modification, were prepared by melt spinning in selected conditions, such as low temperature of the melt and controlled cooling in the sub-spinneret zone. The physical modification was conducted by thermo-mechanical processing, resulting in hollow fibre membranes with microporous or with asymmetric structure. A major factor affecting the membrane structure after physical modification consisted of the sub-spinneret zone cooling conditions during melt-spinning. Gas permeabilities of O-2, N-2 and CO2 were measured and adsorption porosimetry was used to estimate the hollow fibre membrane＇s structure.