The diffusion mechanism of noble gases in the alpha form crystal of syndiotactic polystyrene was investigated using a molecular dynamics simulation. The diffusion process was analyzed in connection with the cavity structure. A capillary-like cavity structure was clearly found in the alpha form. The efficiency of the gas diffusion in the capillary was much higher. The diffusion coefficients were more than one order higher than those in amorphous polystyrene. The anisotropy of diffusion becomes more significant for the larger size gases, whose diffusion in the alpha form can be effectively described by a one-dimensional transport. (C) 2003 Elsevier Science B.V. All rights reserved.