This work presents the results of the laboratory research on the impact of grain size composition of raw mixtures on properties of heat processed products when implementing the compression molding method. We have analyzed opokas of several fields: Zhuravksy, Shevchenkovsky and Bekeshevsky. Grain size composition of ground materials was characterized by a greater grain size of 0,315; 0,63; 1,25 and 2,5 mm. The link between degree of grinding and caking index, water absorption, mean density, compression and bending resistance was established. The conclusions based on the research results revealed the impact of grain size composition on properties of ceramic products and application potential of opokas as a feedstock in the ceramics industry.