Abstract The main purpose of this study was to examine whether the use of models in the teaching of two digit numbers develop students’ conceptual understandingof addition and subtraction operations. Consequently, Quasi-Experimental Single –Group Interrupted Time-Series Design research was chosen as an appropriate research design for achieving the research aim and Realistic Mathematics Education (RME) was used as a guide to design the lesson. This studyused both qualitative and quantitative research approaches. The study was conducted in Eweket Fana primary school specially grade one ‘D’ students. Then, the participants of this study were selected randomly from all first grade four sections.The experimental class room had fifty students with twenty eight female and twenty two male students. Lesson Observation, Lesson Evaluation, Interview, Focus Group Discussion and tests were the main instruments used in collecting data for the study. The result of this study shows that the beads string could bridge students from the contextual problems (taking grains, beads and making grains bracelets) to the use of the number line. It also shows that the number line could promote students to use different strategies in solving addition and subtraction problems. Based on these findings, it is recommended to apply RME in the teaching learning process to make it more meaningful for students. Keywords:Addition,Subtraction, Design Based Research, Realistic Mathematics Education, the StringBeads,Number Line