There is a need for further investigation of evidence-based practices for students with autism spectrum disorders and developmental disabilities leading to students' success within the general education curriculum (Cihak & Foust, 2008; Rockwell, Griffin, & Jones, 2011; Schaefer-Whitby, Travers, & Harnik, 2009). The purpose of this study was to investigate the concrete-representational-abstract instructional sequence and the Strategic Instruction Model (CRA-SIM) with regard to mathematics computation performance of students with ASD and DD. Eleven elementary students with ASD and DD participated in four weeks of instruction with regard to basic additional and subtraction using CRA-SIM. A paired samples t test statistical procedure was conducted to evaluate the differences in student progress across curriculum-based measures over the course of the study. Results indicated that the students made significant progress across three CBMs administered over the course of the study. Measures of effect size also indicated that CRA-SIM had a strong effect on student performance. These results, descriptive performance results, and their implications will be discussed.