The aim of this study is to investigate the organic petrology of shale in Phnom Mreach, Angkor Chum District, Siem Reap province, onshore Cambodia. Thin-section is carried out to analyze mineral compositions, sedimentary texture, and shale classification. Polish-section is used to identify the types of organic matter under reflected light. Scanning electron microscope (SEM) is analyzed for further detailed study on types and morphology of organic matter. The results showed that shales are mainly composed of quartz, clay minerals, calcite, mica, and small amount of plagioclase. Moreover, shales are classified into silica-rich argillaceous mudstone and clay-dominated lithotype. These shales consist of parallel and wavy lamina-structures. Furthermore, there are three types of organic matter are identified under SEM and polish section, such as vitrinite, inertinite, and alginite. Moreover, the organic petrology of shales can contribute to better understanding sources of organic matter. The presence of vitrinite and inertinite is suggested that organic matter has a source from woody tissues and plant materials that belong to a humic-type deposit.