Abstract: The regular mass spectrometric pattern of the TMS derivatives of sterols was studied using mass spectrometry and some TMS derivatives standards. The number of double bonds, methylizes, alkylizes and their related position in the sterol nuclear and the side chains were confirmed by the characteristic ions. Total lipids were extracted using Bligh-Dyer method, sterols were isolated by solvent partition with V(chloroform):V(hexane) =1:4, derivated by BSTFA, and analyzed by GC/MS. The regular pattern was used for identifying the sterols in four dinoflagellates from the Symbodinium sp., Scrippsiella trochoidea, Prorocentrum micans and Gyrodinium instriatum.