Abstract: The methods of acid-base separation and extraction-chromatography were used to separate Longkou shale oil into acid fractions, basic fractions and five neutral fractions. The molecular structure and mass distribution of oxygen compounds in Longkou shale oil were investigated using gas chromatography-mass spectrometry (GC-MS) and negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results of GC-MS show that the oxygen compounds in acid fractions involve phenols, indanols, naphthols, phenylphenols, fluorenols, phenanthrenols, and carboxylic acids ranging from C₅ to C₁₆. The oxygen compounds in neutral fraction 4 and 5 involve aliphatic ketones ranging from C₉ to C₃₂ and lipid compounds. The results of ESI FT-ICR MS show that the class species of O₁, O₂, O₃, N₁O₁, N₁O₂ are found. The O₁ and O₂ have the higher ion intensity among the species in shale oil. The m/z range of shale oil is from 150 to 600. It is obviously that Longkou shale oil is mainly composed of small molecular compounds which are polymerized by Van der Waals force and hydrogen bonding. The properties of shale oil are similar to macromolecular compounds. Longkou shale oil has O₁, O₂, O₃ compounds mainly with DBE of 1 and 4, suggesting that Longkou shale oil can be regarded as a less biodegraded oil.