Shale oil has attracted more attention, as a very important substitutable fuel resource. In the present research, the classes and structures of nitrogen species in hydrotreated and untreated Fushun shale oil (FSO) are characterized by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Experimental results have demonstrated that most of the nitrogen compounds in FSO are removed effectively during the hydrotreatment. N-1 and N-2 classes are dominant in FSO, and their structures are deduced in terms of the double bond equivalent (DBE) values and the Fourier transform infrared (FTIR) spectra. N-1 class species in FSO are probably pyridines, indoles, carbazoles, benzocarbazoles, and their derivatives. After hydrotreating, the N-1 class species in hydrotreated Fushun shale oil (HFSO) extend over a wider range of DBE values and carbon numbers than in the original FSO. It can be concluded that the NI class species in HFSO are generated from compounds containing two or more heteroatoms, such as N-2, N1O1, N1O2, N1O1S1, N1S1, and N2S1 class species.