This paper presents a centerline-based parametric model of colon for collision detection and visualization of the colon lumen for colonoscopy simulator. The prevailing marching cubes algorithm for 3D surface construction can provide a high resolution mesh of triangular elements of the colon lumen from CT data. But a well organized mesh structure reflecting the geometric information of the colon is essential to fast and accurate computation of contact between the colonoscope and colon, and the corresponding reflective force in the colonoscopy simulator. The colon is modeled as parametric arrangement of triangular elements with its surface along the centerline of the colon. All the vertices are parameterized according to the radial angle along the centerline, so that the triangles around the viewpoint can be found fast. The centerline-based parametric colon model has 75,744 triangular elements compared to 373,364 of the model constructed by the marching cubes algorithm.