Abstract: Considering the tree-like traits of trachea and bronchus, an improved region growing method is presented for their segmentation. Getting the initial region of trachea from the top of CT images, the region growing method is improved taking advantage of the correlation between the geometrical and gray-level characteristics of trachea or bronchus in adjacent CT images. Then, the improved method is applied to the search for other regions of trachea and bronchus step by step along the tree-like paths in 3D space and, consequently, all of the data of trachea and bronchus are obtained. With its high automatability, the method fits the segmentation of not only the trachea and bronchus but also the other small regions such as bones and blood vessel system.