The oil yields of oil shale with different density grades are different, which depend on not only the content of kerogen but also the structural characteristic of kerogen in the oil shales. However, no studies focus on the structural characteristics of kerogens in oil shale with different density grades. Therefore, the structural characteristics of kerogens isolated from Longkou oil shale with different densities (1.3-1.4,1.4-1.5, 1.7-1.8, 1.9-2.0 g/cm(3)) were studied by C-13 NMR, FTIR, XRD and XPS. The structural characteristics of the kerogens were studied from three aspects: aliphatic structure, aromatic structure and heteroatom species (O, N and S). The results show that with increasing the density of corresponding oil shales, the ratio of aliphatic carbon and average methylene chain length of the kerogens increase, and the substitutive degree of aromatic ring increases, while the ratios of aromatic carbon and average aromatic cluster size decrease. The aromatic structures in the studied kerogens are mainly 1 and 2 rings. A small amount of aromatic clusters with more than 2 rings are found in the kerogen isolated from the oil shale with the density grade of 1.3-1.4 g/cm(3). The oil yield from Fischer Assay is the highest when the density of oil shale is 1.4-1.5 g/cm(3). In addition, the organic oxygen exists as C-al-O (alcohol and ether), C-ar-O (phenol and ether), O=C-O and C=O and the orderings for the amount of these oxygen functional groups are different in different kerogens. The content of amino nitrogen decreases while the content of chemisorbed nitrogen oxides increases with increasing the density of oil shales. The organic sulfur exists as aliphatic sulfur, aromatic sulfur, sulfoxide and sulfone in the studied kerogens. As the density of oil shales increases, the contents of aliphatic sulfur and sulfone increase while the contents of aromatic sulfur and sulfoxide decrease in their kerogens.