Bohai heavy oil was separated into asphaltene,resin and remaining fraction by precipitation and silica gel chromatography.Molecular compositions of Bohai heavy oil and its fractions were characterized by Fourier transform ion cyclotron resonance(FT- ICR) mass spectrometry(MS) with negative ion electrospray ionization(ESI).The results showed that less polar molecules containing N1,N1O1,O1,O2 compounds with low double bonds equivalent(DBE) in remaining fraction could be ionized by ESI.The heteroatom classes and the distribution patterns of N1,O1,O2 class species in the resin fraction and crude oil are similar, and the heteroatoms classes contain N1,N1O1,N1O2,N2,N2O2S1,O1,O2 classes. The asphaltene fraction contains acidic compounds with high degree of condensation and high order heteroatom,such as N1O3,S1O3,N2O1 and O3 species, which have high interfacial activity.They could promote the reduction of interfacial tension and the strength of interfacial film through adsorbing at oil-water interface.N1O1,N1O2,N1O3 class species could be the oxidation degraded products from N1 compounds, and the N2O1 class species could be generated by oxidizing degradation of N2 compounds. And the polarity of degraded products increases obviously with the increase of oxidizing degraded degree.