Reading in a second language is an interesting area of research because the factors affecting reading have brought about much controversy in related theory and research. Particularly, schema theory has raised long-term debate about whether background knowledge facilitates or impedes reading comprehension. In recognition of such issue, the current research was conducted to examine the impact of background knowledge on second language reading comprehension. Additionally, the impact of time constraint on reading comprehension was also investigated. Thirty-one students of intermediate level of English in Le Quy Don high school, Vietnam took part in the study. Four cloze texts, two of familiar topics and two of unfamiliar topics, were administered under the conditions of limited time and unlimited time. The results revealed significantly positive effects of background knowledge and no time constraint on second language reading comprehension. Further, a significant interaction between background knowledge and time constraint was found. These findings have important implications for second language pedagogy in view of standardized and classroom assessment of reading performance.