Abstract: Energy resolution-mass spectrometry was applied to the study of fragmentation curves of the individual monomer and mixtures of monomers of isomers of chloro-substituted and fluoro-substituted 4(3H)-quinazolinones. It was shown that the fragmentation curves showing changes of ionic intensity ratios of fragment ions of m/z 145 to m/z 125 of monomers or monomer mixtures of the chloro-substituted isomers, and of ionic intensity ratios of fragment ions of m/z 136 to m/z 108 of monomers or monomer mixtures of the fluoro-substituted isomers as plotting against the values of different collision energy in the range of 0.8-1.5 V were found to be closely related with the positions substituted by halogens in molecules of the isomers. Based on the obvious differences among the fragmentation curves, isomers of the halogen substituted 4(3H)-quinazolinones could be identified successfully.