This study investigates the relationship between prior knowledge and reading comprehension in second language among postgraduate students in UPM. Participants in the study were 20 students who have the same level in English as a second language from several faculties. On the basis of a prior-knowledge questionnaire and test, students were selected; they were asked to sit a two-passage reading comprehension exam. According to the questionnaire and the short prior quiz, students had high prior knowledge in one of the two passages, and low prior knowledge in the other. The result showed significantly high relationship between the high prior knowledge and reading comprehension. However, the results showed significantly low relationship between low prior knowledge and reading comprehension. Yet the performance of students in a reading comprehension with high prior knowledge was significantly better than reading comprehension with low prior knowledge.