In this study we aimed to produce AZ63 magnesium alloys containing different amounts of CaO, to investigate the nanohardness behaviour of the resulting alloys. These alloys were produced by mechanical alloying under argon atmosphere. Magnesium based alloys with the initial CaO content of 0.1%, 0.3%, and 0.5% were produced by highenergy ball milling, followed by process that involved cold pressing and sintering. These alloys were characterized using scanning electron microscopy, scanning probe microscopy, X-ray diffraction, and nanoindentation methods. Unloading segments of nanoindentation curves were analyzed using Oliver-Pharr method. Experimental results show that measured nanohardness exhibits a peak load dependence. As a result, in these alloys the microstructure and nanohardness depend on the content of CaO.