The present study investigates the effects of extensive reading on developing the language knowledge and reading abilities of high school learners of English as a foreign language (EFL) over the course of one school year. Participants were divided into control (n = 205) and experimental groups (n = 200), with the former receiving regular grammar instruction and the latter receiving extensive reading instruction. Pre- and posttests consisting of a grammar and vocabulary section and a reading section were conducted to test the effects of the extensive reading program. The study further divided the participants into three proficiency groups. The results showed that the scores of the middle- and lower-proficiency groups increased dramatically compared to the higher-proficiency group. These findings suggest that the participants activated the knowledge they had learned in junior high school through the one-year extensive reading course, resulting in the improvement of their language knowledge and reading abilities.