The purpose of this study was to examine whether elementary music teachers improve their professional knowledge of assessing music learning through developing teaching portfolios. This is a case study with a multiple-case design. Two music teachers who teach general music in Taichung City were participants. The focus of this study included the contents of teaching portfolios, process of developing portfolios, and changes in professional knowledge of assessing music learning by the participants, as well as participants' comments on developing and using portfolios. The data collection approaches include interviews, participation observation, document analysis, and field notes written by the researcher. The findings of this study are (1) the contents of the portfolios developed by the two music teachers differed; (2) developing such portfolios, can help the two music teachers complete their multiple assessment aspects, enhance their reflection, improve their professional knowledge, and increase the interaction and feedback between teachers and students.