The surface characterization of poly(methyl methacrylate)-graft-poly(ethylene oxide) (PMMA-g-PEO) was investigated by XPS and contact-angle measurement, and its in vitro blood compatibility was assessed by plasma recalcification time measurement. The surface and bulk composition of different PMMA-g-PEO graft copolymer showed that PMMA segments were always enriched at the copolymer-air interface, but surface enrichment of PEO segments could occur in films of copolymers with longer PEO side chain (M-n of PEG, 3200) and a higher bulk PEO content. Contact-angle studies indicated that the surface hydrophilicity increased as the surface PEO content increased. The contact angles of water on the copolymer decreased linearly with contact time until they reached a balance value theta (e) or 0, and the relationship between theta (from the static contact angle, theta (s) to theta (e)) and contact time(t) can be expressed by: theta = -kt. The proportionality constant k and Delta theta (theta (s) - theta (e)) depended on the bulk structural parameters (the bulk composition and PEO side chain length). The microphase-separated structure of copolymers was also observed by transmission electron microscopy (TEM). The relationship between the surface properties of PMMA-g PEO graft copolymer and its blood compatibility was addressed.