Hydroxy fatty acids and epoxy fatty acids represent the main oxidized fatty acids in plant oils de⁃rived from linoleic acid and linolenic acid under enzymatic , light or thermal treatment. During the heating process,the changes of oxidized fatty acids in plant oil reflect its oxidative stability and the initial changes of oxidation. In this study, the qualitative and quantitative analysis of oxidized fatty acids of 4 different plant oils (rapeseed oil, sun⁃flower oil, perilla oil and linseed oil) was carried out based on high performance liquid chromatography-triple quad⁃rupole mass spectrometry. The results showed that the contents of polyunsaturated fatty acids(LA，ALA) in 4 plant oils decreased, while the contents of most oxidized fatty acids of LA and ALA increased significantly after the oils being heated at 180 °C for 30 min. Principal component analysis (PCA) result showed that the plant oil samples of different types and treatments were clearly separated. The partial least squares discriminant analysis (PLS-DA) re⁃vealed that six potential markers (13-HODE, 13-oxo-ODE, 12,13-EpOME , 9,10-EpODE, 12,13-EpODE, 15,16-EpODE) were identified in the early stage of thermal oxidation of plant oil. This work could provide guideline for the evaluation of the initial stage of plant oil oxidation and the in-depth investigation of the mechanism.