Poly(4-methyl-1-pentene) (PMP) hollow fibers were prepared and fabricated into gas separation or microporous membranes by the melt-spun and cold-stretched method. PMP resin was melt-extruded into hollow fibers with cold air as the cooling medium. The effects of take-up speed and thermotreatment on the mechanical behavior and morphology of the fibers were investigated. Scanning electronic microscope (SEM) photos were used to reveal the geometric structure of the section and Surface of the hollow fibers. It was found that the original fiber had an asymmetric Structure. A "sandwich" mode was used to describe the formation of this special fine structure. And a series of PMP hollow-fiber membranes were prepared by subsequent drawing, and it was found that there was a "skin-core" structure on the cross section of these hollow-fiber membranes. Asymmetric or microporous PMP hollow-fiber membranes could be obtained by controlling posttreatment conditions. The morphology of these membranes were characterized by SEM, and the gas (oxygen, nitrogen, and carbon dioxide) permeation properties of the membranes was measured. The results indicate that the annealing time of the original fiber and the stretching ratio were the key factors influencing the structure of the resulting membrane. (c) 2006 Wiley Periodicals, Inc.