Abstract: In spite of the development of bio-imaging techniques, X-ray imaging remains an important approach in the study of speech production phenomena.However, the tongue contour in X-ray images is usually difficult to extract because of the image noise and low resolution.In this study, we propose an automatic contour extraction method of tongue in X-ray videos.At first, we take a region gradientbased edge detector to find the initial boundary points.As X-ray image is high noise image and tongue is frequently occluded by teeth, there are high ratio outliers in the initial boundary point set.To solve this problem, we propose a cluster-based point-to-point distance ratio filter to remove the outliers, which greatly reduces the iteration times of later RANSAC and B-spline approximation for the final boundary points.The experiments show that the proposed method obtains tongue's accurate contour, and it could be an effective tool for tongue's continuous motion analysis.