The fine composition of volatile oils in a raw peppermint in July were analysed by combined means of programmed temperature capillary gas chromatography/mass spectrometry (GC/MS).20 compounds were separated and identified.The main constituents were (1α,2α,5β)-menthol (16.88%),(1α,2β,5β)-menthol (74.98%),cis-menthone(3.84%) and (2R-cis)-menthone (1.25%).In the whole July mint there were 4 groups of isomeric compound,their molecular formula were C10 H16 (5 components),C10 H18 O (2 components),C10 H20 O (2 components) and C15H24(8 components),respectively.