Primary schools in Korea have offered English classes once a week while the 5th and the 6th graders receive two classes per week. However, the instructional hours have been considered insufficient to improve students' communicative ability and satisfy their needs for English learning. Accordingly, the Ministry of Education, Science and Technology has piloted a new program that increases the instructional hours of primary school English classes for the 3rd to the 6th graders. Under the current system, the 3rd and the 4th graders take English classes. As part of the research on the program, the present study investigated whether instructional hours of English classes would be effective in terms of students' English achievement. The findings show that the students in the experimental group with increased instructional hours generally performed better on the English achievement test than their counterparts.