In this study, the feasibility of gas chromatography (GC) and gas chromatography–ion mobility spectrometry (GC–IMS) systems for detecting peanut oil (PO) adulterated with rapeseed oil (RO) in different ratios was evaluated. In the GC analysis, the oils were categorized on the basis of fatty acid compositions. In the GC–IMS analysis, the oils were distinguished on the basis of volatile fractions. To demonstrate the high quality of the GC–IMS results, principal component analysis (PCA) and cluster analysis (CA) were conducted. The results showed that fatty acid profiles in PO exhibited higher amounts of palmitic acid (10.70 ± 0.18%), linoleic acid (32.87 ± 0.52%), and arachidic acid (2.30 ± 0.05%). RO had higher levels of linolenic acid (7.49 ± 0.36%) and erucic acid (8.02 ± 0.39%), which were not detected in PO. The linear regression analysis showed that the change in C18:2, C18:3, C20:1, and C22:1 content was highly correlated with the adulteration proportion of RO. However, only significant differences were visible in C18:3 and C20:1 when the adulteration proportion was a minimum of 5%. Satisfactory results of adulteration determination were obtained using the GC–IMS combined with the PCA and CA. Each oil type had unique characteristic compounds, and signals of some volatile compounds changed with the increase in the adulteration proportion especially at run times ranging from 100 to 300 s. For example, the intensity of peak signals of 89–92 increased with the addition of RO while peak intensity of 104 to 107 weakened. These individual signals revealed the volatile compounds responsible for the differences between PO and RO and could help to screen PO adulterated with RO. The established PCA scatter plots and CA could correctly differentiate authentic PO from adulterated samples with at least 1% of RO. Consequently, the GC–IMS shows higher efficiency and feasibility compared with the GC and may serve as a basis for the detection of adulteration of various substances.