This research attempted to discuss the impact of mentor teachers' observation and feedback on elementary beginning teacher's teaching effectiveness. The main findings were: 1. The change of beginning teachers' teaching effectiveness was positive.2. The teaching observation and feedback had positive effects on beginning teachers' teaching effectiveness.After the observation and feedback, beginning teachers had a significant increase on ”clarity”, ”diversity”, ”classroom management”, ”goal-attaining” concerning teaching effectiveness of teaching domains. And beginning teachers also enhanced their teaching effectiveness of 13 teaching behaviors to a significant level.3. The results of semi-structural interviews supported the effects of the observation and feedback.The teaching observation and feedback had positive effects on beginning teachers' teaching effectiveness; namely, beginning teachers progressed pretty well on instructional clarity, diversity, effective communication, classroom management and goal-attaining.