Zirconium diboride (ZrB2) is the material suitable for high performance applications. It has high melting point and high hardness value. On the other hand, recently, mechanical activation and mechanical milling have been extensively used for synthesis of advanced materials. In this present research, nanocrystalline ZrB2 was produced using mixtures of zirconium dioxide, boric acid and pure magnesium as raw materials via mechanochemical process. The phase transformation and structural evolution characterization during process were utilized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. A thermodynamic appraisal showed that the reaction between ZrO2, H3BO3 and Mg is highly exothermic and should be self-sustaining. XRD analyses exhibited that the nanocrystalline ZrB2 was formed after 60 h milling time. The results indicate that increasing milling time up to 40 h had no significant effect other than refining the crystallite size.