Eggshell biomaterial is generally considered waste, however it can be applied in various areas of research due to its unique properties. In this study, the two-phase eggshell nanopowder was prepared by the process of mechanical activation in a planetary ball mill. The phase transformation from calcite, which is the main component of the ES, into aragonite was studied in detail and the properties of milled eggshell were pursued. The effect of mechanical activation was investigated by means of X-ray diffraction, Fourier-transform infrared spectroscopy, nitrogen adsorption, grain size analysis, scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy. The calcite-aragonite phase transformation was confirmed with the conclusion that after 360 min of milling, 58% aragonite is present in the system. The presence of nanoparticles containing mesopores was evidenced. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.