This study aims to analyze the effectiveness of utilizing graded readers on literacy development of English underachievers in high school. The subjects are five Korean high school 1st graders, and the single subject method was employed for this study. The duration of the entire study was ten weeks and the intervention classes of reading with graded books were conducted once or twice a week. Each session was 50 minutes on average. During the intervention period, five students learned English vocabulary and grammar and read texts with two selected graded books. Changes were observed in their literacy skills and affective domains during the intervention period. The results demonstrate that the five underachievers showed meaningful improvements in acquiring words, grammatical knowledge, and reading comprehension. Additionally, the results also indicate that English intervention with reading graded readers had a positive effect on affective factors such as confidence, interest, participation, and anxiety. Pedagogical implications are discussed for English underachievers from elementary to secondary schools.