Poly(4-methyl-1-pentene) was used to prepare porous structures by a thermally induced phase-separation method. Different porous structures were obtained with poly (4-methyl-1-pentene), which has excellent properties as a polymer, and diphenyl ether as a diluent. The affecting factors, including the polymer concentration and cooling temperature, are discussed. Scanning electron microscopy images and porosity values were obtained to investigate the affecting factors. According to the cloud-point temperature and crystallization temperature, a phase diagram was also obtained to explain the phase-separation process. (C) 2009 Wiley Periodicals, Inc. J Appl Polyin Sci 112: 1271-1277, 2009