Abstract: The predominant mineral composition of Huadian oil shale and semi-cokes, including quartz, calcite and clays, was investigated in detail using XRD and SEM. The composition of semi-coke indicates that the minerals change slightly during the pyrolysis of oil shale. The quartz and feldspars remain the same. Few calcites are decomposed to form CaO which reacts with sulfur, resulted from decomposition of pyrite, to form CaS. The amorphous SiO₂, generated from the decomposition of clays, tends to react with other metallic oxides to form relative low melting point mixture. This results in the molten vesicular microstructure in the semi-coke.