This study investigated the effects of Reading Strategy Instruction (RSI) on reading performance and attitude toward reading strategies while reading texts of different difficulty levels. Fifty-five university students studying Political and Basic Sciences took part in this study. After homogenizing the participants, 24 students were in the experimental group and 24 students in the control group. An appropriate text and a higher difficulty level text as well as a questionnaire about attitude toward reading strategies were employed as pre-tests and posttests. The experimental group received RSI through CALLA model, but the control group was taught reading focusing on vocabulary and grammar. Results revealed the experimental group outperformed the control group in reading performance and attitude toward reading strategies in both reading tests. However, RSI was considerably more effective in improving both reading performance and attitude toward reading strategies when students read the test at an appropriate difficulty level. It is recommended for cognitive and affective improvements that texts which are more appropriate to students' reading proficiency level, and not much beyond that, be selected and taught.