Abstract: The EI mass spectra of twenty-five substituted cyclopropanes were studied by using high resolution mass spectrometry and metastable ion measurements to elucidate the fragmentation pathways. In the EI spectra of trans-3-ethynyl-trans- 2-benzoyl- 1-substituted phenyl cyclopropanes (1～7) the molecular ion peaks were very weak, the characteristic ion peaks observed were M-COC_6H_5~+, M-CHC_6H_4X~+, C_6H_5CO~+ and M-CO~+ with rearrangement of phenyl group. For trans-3-trimethyisilyle-thynyl-trans-2-benzoyl-1-substituted phenyl cyclopropanes (8～15), main decomposition pathways involve loss CH_3, COC_6H_5, CHC_6H_4X and Me_9SiOH with the rearrangemet of trimethylsilyl group from carbon atom to oxygen atom. It has been found that the spectra of trans (or cis)-3-trimethyl- silyl- trans- 2- benzoyl- 1- substitutedphenyl cyclopropanes (16～25) shows the intense molecular ion peaks and the characteristic ion peaks M-Me_3SiO~+, M-COC_6H_5~+ and M-CH_3~+. The intensity of molecular ions and some fragment ions of all compounds (1～25) was closely related to the electronic nature of substituted group X.