A teacher professional identity scale was developed for Hong Kong in-service teachers to measure the professional identity of teachers. Most studies of professional identity have been qualitative. The present study tried to examine this important concept using a quantitative method. Based on various studies, one of the ways of understanding the professional identity of teachers was through teachers' roles and practices. This study described the procedure for developing a professional identity scale according to the practices of professional teachers. The final version of the scale included 19 items. Overall, this scale was found to be valid and reliable for measuring the professional identity of Hong Kong in-service teachers in both pilot and main studies. (Contains 6 tables and 2 figures.)