The purpose of this study was to examine the effects of a number line intervention with supported self-explanation on student understanding of fraction magnitude and quality of explanation. Participants were three U.S. middle school students with significant behavior problems. Participants were given eight lessons containing explicit instruction for fraction number line placement and self-explanation. A multiple-probe design was used to evaluate the effects of the intervention. Visual analysis and Tau-U showed the intervention was very effective for improving student placement of fractions on the number line and increasing the quality of student explanations for their answers. Discussion of results and implications for future research and practitioners are included.