Reading comprehension is an important skill for EFL learners with fewer opportunities to communicate and gain knowledge in English. For a complete comprehension, the interaction between the student's prior knowledge and the content is necessary. Background knowledge plays the most important role in comprehension. The present study used a mixed method design to investigate the effect of THIEVES as an activating (schema activation) strategy on EFL learners' reading comprehension. Participants were 63 advanced students from an English language institute in Iran. First, they took the Oxford Quick Placement Test (OQPT) and then they were divided into experimental and control groups and the strategy was administered for the experimental group. Second, they were asked to fulfill a questionnaire about their difficulties through reading comprehension process. Based on the results of the questionnaire analysis, EFL learners confronted problems such as difficult content, unknown vocabulary, lack of background knowledge, and shortage of time. Results showed that there is a significant statistical difference between the scores of the control group and experimental groups. Besides, in the questionnaire learners of the experimental group indicated that THIEVES can help them to overcome their comprehension problems. Thus, it can be concluded that using THIEVES is beneficial in comprehension of expository texts.