Mathematics and Science classes in schools have become a focus to be considered in terms of educational systems and administration around the world in the last decade. Related to the mentioned classes, there are many benefits that lead students to academic success. In the recent years, educators have found that there are so many different factors that effect students' performance in science and math classes. Especially reading comprehension has changed so many traditional procedures in teaching math and science. It also shows remarkable benefits. This research focuses on the effects of reading comprehension on mathematics and science achievement. Students' academic performance on the mentioned classes and their motivation towards those courses will also be the focus of the research. The research is based on the data gathered from the latest PISA results and the opinions of secondary school teachers and students. Findings of this research indicate that there is a correlation between reading comprehension results and student success in math or science classes. It also indicates that reading comprehension contributes positively or negatively to the success results in math or science classes.