Five major forms (lamellar, banded, crack-like, disseminated and segregated encrustation) of organic matter distribution in source rocks have been revealed under scanning electron microscope by using the heavy metal staining technique. The degree of organic impregnation is related to the amount of liquid hydrocarbons in the rocks, and from this relationship a rough estimation of organic matter can be made on the basis of electron microscopic observations. In conjunction with experimental studies it has been found that the distribution forms of organic matter are a function of its maturity in the process of thermal evolution and accordingly some microscopic criteria can be developed for the assessment of source rocks.