Loops concept is one of the basic programming concepts. Students have difficulties in learning loops concept. Helping learners understand loops is an important task. Visualization is one of the ways to help students improve their understanding. The aim of the study was to construct and evaluate a visualization based instruction related to loops. A mixed method study was conducted. In the experimental phase of the study, the effect of visualization based instruction on pre-service teachers' achievement, perceived learning and programming attitude was examined. In the qualitative phase of the study, the main purpose was to get more in depth data related to experimental phase. Visualization based instruction helped pre-service teachers improve their understanding of loops concept.