11 samples of Tertiary argillaceous hydrocarbon source rocks in the Dongying Depression were selected and separated into three fractions according to density difference (<1.6 g/cm3, 1.6~2.2g/cm3 and>2. 2 g/cm3) after crushing them in order to extract the free organic matter, inorganic minerals and organo-clay complex from the argillaceous hydrocarbon source rocks. The quantities and TOC of different fractions were tested and observed by scanning electron microscopy to explore the differences of the occurrence forms of organic matter and the modes of organic carbon distribution in different density fractions. After the density separation, the recovery rate of the quantity and the organic carbon of different fractions approximately reached 100%. These features show credibility of the density separation method and less destruction to hydrocarbon source rocks.