The effect of NH3 plasma treatment on glassy poly(methyl methacrylate) (PMMA) membranes on the diffusion process for penetrant gases (CO2, O, and N-2) was investigated from mean permeability data. The mean permeability coefficient for CO2 definitely depended on the upstream pressure, whereas those. for O-2 and N-2 remained constant regardless of the upstream pressure. For O-2 transport, the permeability increased a little with increasing treatment power, and for N-2 transport, it was not affected by the treatment power. For CO2 transport, NH3 plasma treatment promoted the transport of Langmuir mode, presumably through an increased Langmuir capacity constant for CO2, NH3 plasma treatment for PMMA membranes resulted in an increase in the separation factor of CO2 relative to N-2 and in the permeability to CO2.