This paper examines Chinese language teachers’ developing confidence in the application of effective principles for online teaching in Mandarin Chinese. Twelve in-service secondary and post-secondary Chinese language teachers in the United States completed training in an intensive summer program that was followed by teaching in the local classrooms in fall. They completed a self-assessment checklist on the application of effective principles for online language teaching at three stages: (1) pre-program in summer, (2) post-program in summer, and (3) post-program in fall. The study employed a quantitative method to document teachers’ acquired knowledge and skills based on the results of the self-assessment checklists. Qualitative analysis based on teachers’ multifaceted reflections complements quantitative analysis. The results indicate that teachers’ confidence in the skill sets required for the application of effective principles and strategies for online teaching increased significantly after the summer training, and their confidence remained high or showed further improvement in the majority of the skill sets upon the completion of teaching practicum during the post-program stage. Limitations and directions for future research are discussed at the end of the paper.