Abstract Plasma graft onto microporous polypropylene (PP) membranes was studied. PP substrates had 0.45 μ of average pore size and 80 to 150 μ thickness and monomer for graft was N, N-dimethylacrylamide. Even by short exposure to argon and hydrogen plasmas less than 10 s, post graft polymerization was very fast. Grafting layer could be distinguished from relatively less-grafted portion by electron microscopy. However, graft was, more or less, noticed to form over cross-section of substrate. Plasmas were excited by 13.56 MHz radio frequency source and the wattage less than 10 W was usually enough to prevent PP from heat damage. Grafting rate was dependent on plasma-exposure time. For argon plasma at 10 W, 0.1 Torr (13.3 Pa), grafting rate decreased after maximum rate was observed at 10 s exposure. Analysis by electron spin resonance (ESR) revealed that relationship between spin concentration and irradiation time was somewhat different from reported data on polyethylene substrate. Alkyl radicals showing an eight-line-signal on ESR spectra were rapidly converted to peroxy radicals in air at almost 100%-yield. Both kinds of radicals could initiate graft, and alkyl radicals were found more active. Apparent activation energies were estimated to be 14.2 and 7.9 kcal/mol for graft polymerizations initiated by peroxy and alkyl radicals, respectively. Thermal analysis and X-ray diffraction revealed that graft may exist not only inner and outer surfaces but also in bulk region of substrate. For substrates more than 1000% grafted, even crystalline region was destroyed completely. Diffusion or absorption of monomer into bulk may be an important factor to support growth of graft polymer.