Asymmetric poly(4-methyl-1-pentene) (TPX) membranes were prepared via a two-stage wet phase inversion method to improve their gas Bur. The effects of solution and coagulant on the morphology of membranes were studied from the view point of thermodynamics and diffusion kinetics. It is also found that the surface tension and solubility parameters of second-bath media play important roles in determining the membrane morphology and the gas separation performance. In addition, the effect of milling temperature and the decline of gas flux with storage time are discussed.