This study examined the effect of self-regulated learning on sixth-grade Turkish students' mathematics achievements and motivational beliefs. Both quantitative and qualitative research methods were used in the study. Participants included sixth-grade students attending at TOKI 125. Year Middle School in Nevsehir (Turkey) during the 2014-2015 academic year. Two classes of the school were randomly appointed as experimental group (6B, 22 students) and control group (6C, 23 students), respectively. The experimental process went on for 12 weeks. Instructional activities developed according to Zimmerman's cyclical model, one of the self-regulated learning models, were applied with experimental group students, meanwhile regular curriculum of MEB (Ministry of National Education) was delivered to control group students. The sub-dimensions of self-efficacy and goal orientations were included in the research under motivational beliefs aspect, which is one of the most essential aspects of self-regulated learning. The quantitative findings indicated a significant difference in favor of experimental group with regard to mathematics achievements and motivational beliefs. The qualitative data obtained through interviews and document analysis during and after the experimental process further revealed that students started to see math as fun, like it more, grasp the importance of it in everyday life, build more self-confidence, set specific targets for themselves, and monitor their learning process.