Abstract: Chemical conversion of a compound to an appropriate derivative can some-times improve the resulting mass spectral data, either by increasing the compound’s vapor pressure or by making its spectrum more easily interpretable. As a kind of important derivative reagent, the attribution of mass spectrum peak in trifluoro -N-methyl-N -(trimethylsilyl) acetamide (MSTFA) has a great significance to further investigating its reaction mechanism. Through studying the fluorinated and non fluorinated derivatives as well as fluorinated aromatic phenolic compounds by gas chromatography mass spectrometry (GC/MS), it was found that fluorinated derivatives have characteristic m/z 77 even electron fragment ions, and non fluorinated derivatives can not form this characteristic ions. High resolution mass spectrometer confirms above characteristic ions is C₂H₆FSi⁺, indicating skeleton rearrangement has an important effect on the formation of m/z 77 fragment ions. Therefore, the formation way may be unimolecular nucleophilic reactions (S_N1): oxygen (nitrogen) is intermediate in its ability to influence either α or i reactions. In an aliphatic ether attraction of an electron pair initiated by the localized positive charge on the oxygen (nitrogen) can form the trimethylsilane ion and the alkoxyl radical. First molecular ion by α-cleavage loss of methyl radical at a reactive site, positive charge localized on the oxygen (nitrogen) atom. Because the positive charge attract a couple of electrons migrate, the charge site is moved localized on the silicon atom, then fluorine atom carries a couple of electrons nucleophilic attack on the silylium ion, and O(N)—Si bond’s a couple of electrons migrate, and nucleophilic elimination reaction in intramolecular is occur. The mechanism of these fragmentations has been studied with the GC/MS analysis and deuterium labeled analogs whose preparation is reported.