Toll evasion is a known challenging problem in expressway transportation. Existing solutions mainly rely on the audit by people on the spot. However, with the increase of expressway networks, this kind of methods become less practical and lead to low efficiency. Considering this deficiency, this paper presents a method based on data mining to automatically assist the toll audit. Our method not only takes use of the data recorded from the expressway toll system, but also brings convenience to the auditing process.