Abstract: To investigate the characteristics of mass spectra of four sildenafil derivatives, their fragments were analyzed using electrospray ion trap mass spectrometry in a multi-stage MS full scan mode performed on a Finnigan LCQ instrument. The mass spectrometry were operated in the positive mode. The mass spectra were obtained mainly by analyzing the fragments of the four compounds. The full scan MS2 spectra of each compound gave characteristic fragment ions formed through cleavage of the S-N, S-C, C-O bond between the pyrimidine ring and the aromatic ring, the cleavage of the C-S bond after the arrangement of sulfuryl group, respectively. The base peak of the MS2 spectra was formed though the C-O cleavage. The full scan MS3 spectra gave characteristic fragment ions obtained after elimination of carbon monoxide, ammonia, acidamide and undergoing the retro-Diels-Alder reaction in pryrazole-pyrimiding rings, respectively. These character-istics can be used for future structure elucidation in studies of analogues of sildenafil.