Since the teaching of English was introduced into the primary school as a required subject in 1997, no achievement test has been administered for the purpose of comparing English achievement of primary school students who are receiving private instruction with that of those who are not. The purpose of this study was to investigate what impact private instruction had on English achievement of third- and fourth-year students in the primary school. For this purpose, an English achievement test was developed based on the 7th national curriculum and two questionnaires were administered: one questionnaire among 743 third- and fourth-year primary school students and the other among 22 primary school teachers. The results showed that students who were receiving private instruction outperformed those who were not on the English achievement test and that teachers had difficulty in handling English class due to the discrepancy of overall English proficiency between tutored students and non-tutored students. The findings suggest that teacher training needs to be intensified to ensure quality English language teaching in the primary school.