The effect of the processing conditions of dolomite powders on the antiviral activity was studied against H5N3 avian influenza virus. When dolomite was fired in the ambient atmosphere, it decomposed into MgO and CaO by two stages at different temperatures, namely into MgO+CaCO3 at ca.1010 K and MgO+CaO at ca. 1070 K. Strong antiviral activity was observed after the complete decomposition into CaO and MgO. However, when the firing temperature exceeded 1673 K, the antiviral activity was degraded. It was also found that the degraded antiviral activity was recovered by sever ball mill grinding.