Despite the general understanding of the prominent role of basic reading sub-skills, such as decoding abilities and reading fluency, in the reading comprehension of younger and proficient first and second language readers, little is known about the role of these skills in older and less skilled readers in foreign language contexts. The present study was designed to investigate how decoding skills and reading fluency relate to the reading comprehension of such readers, and to identify the relative strengths between the two. Two hundred twenty two Korean high school students participated, and their performance on decoding, reading fluency, and reading comprehension tasks was analyzed. The results showed that both decoding skills and reading fluency were strongly correlated to reading comprehension. Moreover, between the two fundamental reading sub-skills, decoding skills outperformed reading fluency in explaining reading comprehension, making a unique contribution beyond the effects of reading fluency. These findings suggest that a basic skill like word decoding still deserves instructional attention even at the secondary schools in EFL contexts.