This study investigates the effects of the text difficulty of extensive reading materials on the reading comprehension and reading motivation of English as a foreign language (EFL) vocational high school students in Taiwan. Two experimental groups were assigned, on an individual basis, to read graded readers at either one level below (‘i-1’) or one level beyond (‘i+1’) their current level, while a control group followed their regular curriculum. The results showed that after treatment, the ‘i-1’ group improved their overall comprehension and the subset of literal comprehension. They also outperformed the ‘i+1’ group on the same measures. For reading motivation, the ‘i+1’ group’s overall motivation was promoted. Both groups enhanced their reading engagement, while only the ‘i-1’ group inhibited reading avoidance. Moreover, the ‘i+1’ group outperformed the ‘i-1’ group in the perception of self-efficacy. Overall, the ‘i-1’ level yielded better effects on reading comprehension; the ‘i+1’ level, on reading motivation
