Online teacher professional development is widely regarded as a promising way to improve the quality of teachers and the effect of teaching. Micro-learning, which has the potential to fit the learner's personalized learning needs through a variety of small educational resources and flexible learning opportunities, could be an efficient way for the online teacher professional development. However, the scattered knowledge points and fragmented learning time in micro-learning may limit the learner's comprehension and the construction of knowledge. In this study, a knowledge map-based online micro-learning system was developed to solve this problem. To verify the effectiveness of the proposed system, a quasi-experiment was conducted during a three-week online course for teachers' professional development. A total of 42 in-service Mandarin teachers participated in the course, and were divided into two equal groups. An experimental group learned by using a knowledge map tool while a control group learned the same contents in the form of the conventional list type. The results indicate that the experimental group outperformed the control group in terms of learning engagement, and showed a clearer and more complete knowledge structure. Furthermore, micro-learning coupled with knowledge maps was found to improve the teachers' learning performance.