The thermal decomposition of three methyl anisole isomers (o-, m-, and p-methyl anisole) were investigated to understand the influences of methyl substitution position on the benzene ring at low pressure (below 15 Pa) within temperature range from 473 to 1473 K. The pyrolytic phenomena were studied by using vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (SPI-TOFMS) to identify the intermediates, radicals and final products, and the relative concentration profiles of the pyrolytic products were evaluated by the semiquantitative analysis method. It was showed that the bond homolysis of PhO-CH3 was the initial reaction for all three methyl substituted anisole, and the produced intermediates and final products show similar among the pyrolysis experiments of three methyl anisole isomers. Moreover, the effects of the different substituted position on the benzene ring of the methyl group presented on the extraordinary differences of the maximum relative concentration of m/z 106 (6-methylen-2, 4-cyclohexadien-1-one or 4-methylene-2, 5-cyclohexadiene-1-one), The theoretical calculations indicate different generating pathways of m/z 106 for the pyrolysis of m-methyl anisole and o-, p-methyl anisole. Noteworthy, the isomerization reaction played a significant role in the pyrolysis of m-methyl anisole.