Abstract: The contours of wheels play an important role in the traffic accident analysis system. Their detecting precision will influence the measurement precision and the investigation quality directly. A new automatic detecting approach is proposed in this paper. As the contour of a wheel rim is easier to detect than the contour of a wheel, and the contour of a wheel rim and the contour of a wheel are approximately concentric circles on the same plane, we first detect the contour of the wheel rim, the center of the wheel rim and the contacting point between the wheel and the ground, then we detect the contour of the wheel via the invariance of cross ratio under projective transformation. Our extensive experimental results with real images under various weather conditions show that the proposed approach is capable of detecting contours of wheels robustly and automatically.