Abstract: The decomposition reaction of molecular ion of phenol and phenylmethylether had been stuied by molecular orbitals theory. The ionization potential and inner energy of ions and fragments which were produced in decomposition reaction were calculated. The theoretical calculation was performed with semiempirical quantum chemical method AMl. (1) According to the Stevenson law, the positive charge should be located at that fragment whose ionization energy is lower than others. (2) Comparing different reaction, the preferential reaction should he that one in which the reaction enegry is lower than others. In the decomposition processes of phenol and phenylmethylether, the results of theoretical study and results of experiment were identically. The experiment was performed by linked scan of magnetic field and electrostatic fiels of mass spectrometer. The initial fragmentation of phenol and phenymethylether molecular ion in mass spectrometer was interprelated.