Magnesium and its alloys have been used in the recent development of lightweight, biodegradeable implant materials. However, the corrosion properties of magnesium limit its usefulness. In a previous study, a micro-arc oxidation (MAO) method was used to modify a Mg-1.0 wt % Zn-1.0 wt % Ca alloy surface, with the purpose of improving the corrosion resistance of Mg alloys. However, the blood compatibility of MAO-treated Mg alloy is unknown. Results of cytotoxicity assays with bone marrow-derived mesenchymal stem cells showed that extracts of MAO-treated alloy significantly decreased cytotoxicity compared to titanium alloy extract. Results of blood compatibility tests showed that the MAO group had a decreased hemolytic ratio (2.25%) compared to the untreated Mg alloy group (24.58%) (p < 0.001). The MAO group showed significantly shorter prothrombin and thrombin times and significantly longer activated partial thromboplastin time than the untreated Mg alloy group. Arachidonic acid- and adenosine diphosphate-induced platelet aggregations were significantly decreased by the untreated Mg alloy extract, and they were less affected by extract of MAO-treated Mg alloy. In conclusion, MAO-treated Mg-1.0 wt % Zn-1.0 wt % Ca alloy exhibits favorable blood compatibility characteristics and may be useful in the development of magnesium implant materials. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A:, 2011.