This study explored the relationship between two dimensions of vocabulary knowledge, that is, breadth of vocabulary (the number of words known) and depth of vocabulary (the richness of word knowledge), and their effects on different aspects of English reading in Chinese high school students learning English as a second language. Two hundred and forty-six Grade 8 students in China were administered measures of word reading, vocabulary breadth, vocabulary depth, and reading comprehension. Results showed that breadth and depth of vocabulary were moderately correlated. They both contributed to word reading, but breadth of vocabulary had a stronger effect than depth of vocabulary. When reading comprehension was the outcome measure, vocabulary breadth significantly predicted a multiple-choice reading comprehension measure, which requires general understanding of the text, while vocabulary depth contributed to summary writing, a measure of deeper text processing. Discussion focuses on the important roles of different dimensions of vocabulary knowledge for different types of second language reading.