Background: Most residents-as-teachers (RaT) programs are delivered over days to weeks without comprehensive evaluation, and stepwise approaches have rarely been applied to RaT activities. This study aimed to depict the implementation experience and evaluate the effectiveness of a novel longitudinal 3-year, stepwise RaT program. Methods: The longitudinal RaT program included three once yearly face-to-face courses according to the different teaching roles of the residents. To evaluate the effectiveness of the new longitudinal program, we designed a randomized controlled study for firstyear residents of all specialties in one medical center. The effectiveness was evaluated by the objective structured teaching exercise (OSTE), feedback from participants and medical students, and evaluation of clinical practice performance by program directors. Results: A total of 35 (37.6%) of 93 residents participated in this study, and 13 (37.1%) of all enrolled residents completed all 3-year courses, including seven for the longitudinal program and six for the traditional. The serial OSTE revealed significantly higher scores in the longitudinal group in the second and third years (13.43 vs 9.50, p = 0.001 and 14.29 vs 10.33, p = 0.015). Satisfaction was higher when advanced topics were taught in the second and third years compared with those taught in the first year (4.43 vs 3.89, p = 0.02). The feedback from medical students was similar between the two groups, and the evaluation from program directors revealed insignificantly better clinical performance among the longitudinal course participants. Conclusion: It is challenging to conduct a multi-year longitudinal RaT program on young residents. Nevertheless, this longitudinal program was potentially associated with better learning retention and higher satisfaction and worthy to be promoted.