The effect of water content in a dolomite sample on the grindability and change in the crystalline structure during grinding were studied by using a planetary ball mill. The size reduction of the dolomite sample proceeds in the presence of water more than 10 mass%, while, below this content the specific surface area of ground product indicates a downward trend in prolonged grinding after showing an increase at the initial stage. The phase transformation from calcite to aragonite takes place during the grinding of dolomite samples containing over 10 mass% of water, while dry grinding of the sample only decreases the peak intensity of the original X-ray diffraction patterns. DTA reveals that the temperature at decomposition of magnesium carbonate shifts towards the lower-temperature side with the progress of grinding.