The benefits of content writing are widely recognized, yet there are still few studies that evaluate the benefits and challenges of mathematical writing (MW) for students. The National Council of Teachers of Mathematics and the Common Core State Standards for Mathematics have an emphasis on written communication of mathematical reasoning as an integral part of curriculum, peer-reviewed publications on this topic are lacking. This study evaluates the effects of self-regulated strategy development (SRSD) intervention on sixth-grade students' MW. Findings indicate that the intervention had significant impact on MW outcomes. Additionally, the five predictors assessed prior to the intervention accounted for approximately 73.6% of variance in MW quality. Implications for the field and future research are discussed.