Abstract: Recently, the number of children receiving medical treatment because of precocious puberty has increased. One diagnosis method for detecting precocious puberty is a bone age assessment which evaluates a child’s bone age by observing the shape of bones and joints. In this study, we developed a process to extract wrist big Region of Interest (ROI) to help extract 2 assessed bones required for the Tanner-Whitehouse 3 (TW3) assessment. We investigate the test methods and results for the algorithm derived from various tests. Through various methodologies, the wrist big ROI are obtained among which the 13 assessed bones in the wrist required by TW3 are included.