Polymethyl methacrylate (PMMA) was surface modified by tetrafluoromethane (CF4) plasma and its effect on the human breast cancer cells (MCF-7) adhesion and proliferation were investigated. The promotion of MCF-7 cell growth was positively and quantitatively related to the CF4 plasma modification as revealed by different characterizations. From ESCA analysis, it is revealed that CF4 plasma treatment chemically alters the capability of PMMA to adsorb proteins from cell culture medium, which is about four times comparing to that of pristine PMMA. In addition to chemical alteration, CF4 plasma also concurrently delivered physical modification by doubling the surface roughness, as observed by AFM analysis.