This study explores the effects of reading material types in sustained silent reading on Korean EFL learners, focusing on their reading speed and comprehension. Reading materials are divided into four types: longer texts with Korean vocabulary glosses, longer texts without the glosses, short texts, and a mixture of the three text types. For this research, an seven-month-long sustained silent reading program was developed for high school students. The participants were divided into four groups according to the types of reading texts they were to read. The reading rates and reading comprehension were measured before and after the program was implemented. The results were as follows: First, the gain in reading rates was most noticeable in the group reading longer texts with the vocabulary glosses. Second, there was a statistically significant difference in reading comprehension before and after the program was implemented. The group which read a mixture of the three reading text types showed the most remarkable gain in comprehension. The findings of this study suggest the need for further research on the relationship between reading comprehension and reading rates, and how to incorporate sustained silent reading into the reading curriculum.