To reconstruct a tooth model from CT slices, we need to define the accurate contour of each tooth from CT slices. In this paper, we propose a semi-automatic method for drawing the contours of teeth in dental CT images. First of all, we select the reference slice from CT slices and carry out tooth segmentation interactively based on shape characteristic of each tooth. The dental CT images have the following distinct characteristic: in two neighboring CT slices, the size, location and intensity of corresponding tooth is very similar, so we can project the bounding box of the tooth whose contour has been drawn onto the next slice as the operation region of corresponding tooth. And then we perform segmentation operation tooth by tooth in the operation region based on the region growing algorithm using the information passed from the previous slice. With the reference slice as the starting slice, we carry out tooth segmentation slice by slice like this way automatically. The test result shows that the proposed method can detect the contours of teeth quickly and reliably with only little user intervention and it has practical value.