Abstract-The present study aimed at investigating the effects of Short-Term Memory (STM) span, syntactic awareness, and vocabulary knowledge on Iranian EFL learners' reading comprehension. The research data were collected using quantitative methods. The data collection instruments included Oxford Placement Test, a working memory span test, and reading pretests and posttests. Sixty seven female intermediate level students were randomly assigned into the three groups of control, syntactic awareness, and vocabulary knowledge. Their performances then were measured and analyzed. Not only did the results reveal that there was a significant difference between the experimental groups and the control group in reading comprehension but also a significant difference between the two experimental groups. In other words, students in the vocabulary knowledge group benefitted more than students in the syntactic awareness. In addition, findings from the study indicated that STM does not affect reading comprehension ability of the students.