This study aims to explore the Native English-Speaking Teacher (NEST) Program in relation to teachers' instruction and students' learning of English in primary schools in Taiwan. Inviting native English-speakers to teach English in the school system is not an unusual practice in the Asia-Pacific region. As the practice of including NESTs in the school system has become prevalent in Taiwan, alongside other Asian countries, its significance cannot be overlooked. It is worthwhile to look into the impact of NEST programs on local English teachers and students in context. Through interviewing, class observation and questionnaire, this study finds that the NEST Program in Taiwan did not have a considerable impact on teachers' instruction, whereas its effect on teachers' professional development was mainly concerning English language proficiency. As for students' learning, the NESTs in the Program served as a language model in the classroom and were able to arouse students' motivation at the beginning of the school year. Nevertheless, the teachers and the students had unfavorable perceptions of the NESTs due to their unexpected leave without notice. In light of the research findings, the author makes suggestions on the NEST Program.