This study investigated the effects of Native English-Speaking Teachers (NEST) and Non-Native English-Speaking Teachers (NNEST) on students' English proficiency and perceptions. The research methodology employed an observational study based using critical applied linguistic. Data collection was through a mixed method. The tools used were the Cambridge English: Key English Test (KET), classroom observation evaluation forms, and interviews. The participants consisted of 252 upper primary students from one private school in Chiang Mai, Thailand, during the academic year 2019. Results indicated that students' English proficiency was increased significantly at 0.01 level in both groups. The gain score suggested that NNEST can make a higher gain score than NEST in all grade levels. Student's answers show NNEST score a higher agreeability towards teachers' teaching abilities, English abilities, and the creation of an engaging learning atmosphere over NEST. Classroom observations implemented by three English Learning Teachers confirmed the results that NNEST is more agreeable than NEST in teachers' teaching ability and motivating learning atmospheres in classrooms. However, in teachers' English skills, the experts' perceptions were opposite that of the students. Lastly, the interviews with the students reflected three key aspects: their preferences of English teacher advantages, disadvantages, and strengths of both NNEST and NEST.