Virtual colonoscopy (VC) is a safe and fast medical imaging procedure to screen the colon for polyps. And it has become very popular recently. Colon segmentation is a necessary and important step of such an examination procedure. In this paper, an automatic colon segmentation method is proposed. The fluid inside the colon is first identified and removed based on its characteristic of horizontal surface. A simple 3D region growing algorithm is applied to obtain initial segmentation of air, which is served as the basis of the ensuing automatic locating object and background seeds. Then the isolated-connected threshold algorithm, together with the above seeds, is applied to obtain the final results. The colon can be obtained by applying morphological operations to the segmentation results of air. Our proposed algorithm can automatically segment different substances based on the isolated-connected threshold. It allows the user to modify the segmentation results interactively by providing more object or background seeds.