205 Copyright © 2019, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 11 DOI: 10.4018/978-1-5225-5799-9.ch011 ABSTRACT In school, reading literacy is also vital for learning mathematics and development of mathematical literacy. In addition to knowing and understanding numbers, symbols, and relations among them, also large enough vocabulary is needed, and the ability of reading and understanding word problems or mathematical texts. The empirical research is based on the descriptive and causal non-experimental method, with data having been gathered with the support of a test of mathematical knowledge. In the sample, 89 students of the third grade of basic school were involved. The results of the empirical research that examined the relations between the performance of students in mathematics and in Slovenian have shown the students with higher grades in Slovenian have performed better in solving simple and more demanding mathematical problems than the students with lower grades in Slovenian. The Importance of Reading Literacy in Learning Mathematics Amalija Žakelj University of Primorska, Slovenia Mara Cotič University of Primorska, Slovenia Darjo Felda University of Primorska, Slovenia Sanela Mešinović University of Primorska, Slovenia