A rapid analytical method that allows comprehensive detection and structural elucidation of synthetic cathinones was developed using a gas chromatograph/ tandem mass spectrometer (GC-MS/MS) capable of two-stage mass spectrometry. The proposed method consists of three simultaneous analytical procedures: 1) selective detection of the carbonyl group, characteristic of cathinones, using multiple reaction monitoring (MRM) and determination of both 2) iminium cations and 3) substituted benzoyl cations generated by α-cleavage of amine and benzoyl moieties, respectively, using product ion scan. For all cathinones examined, single peaks were detected at the same retention time on MRM chromatograms in procedure 1) and on total ion current chromatograms (TIC) in procedures 2) and 3). MRM in procedure 1) showed a transition of the substituted benzoyl cation > substituted phenyl cation due to CO elimination by collision-induced dissociation (CID), which demonstrated the existence of a carbonyl group in the structures. Each product ion mass spectrum for the substituted benzoyl cation allowed to not only determine the substituted group on the aromatic ring for all the cathinones, but also differentiate the corresponding positional isomers for ethyl, methoxy, and methylenedioxy substitutions, although identification of the substituted position for methyl, bromine, and fluorine groups on the benzene ring was difficult. On the other hand, the difference in product ion mass spectra between structural isomers of iminium cations was significantly clear, leading to easy discriminative identification of the isomers.