A rapid and sensitive method is proposed for determination of peppermint oil adulterated with six edible plant oils using gas chromatography-mass spectrometry (GCMS). With identification of characteristic components of plant oils including linoleic acid, oleic acid, palmitic acid, and stearic acid, it can be determined whether the peppermint oil is spiked with plant oils. Common and special components of six plant oils including soybean oil, blend oil, sesame oil, corn oil, peanut oil, and rapeseed oil are discussed after methyl esterification. The results show that the peppermint oil with more than 0.001% plant oils can be detected. Levels of the adulteration of the plant oils can be obtained based on the peak area ratio between the sum of four kinds of characteristic fatty acid and L-menthol. As a sensitive and reliable method, it provides an effective technique for the quality control of peppermint oil.