The purpose of this study was to investigate the visualization in mathematical generalization process and its importance. To do this, we searched which visualizations were used by the participants in appropriate situations, how they set forth these and what kind of visual images they had. We employed case study technique as a quantitative research method on five participants that were pre-service mathematics teachers. The results showed that the visualization was widely employed by the participants, but in different styles and different visual images were presented in the process. Visualizations had an important place on the relations between concepts, had observable effects on the development of the processes. In the light of the results, some suggestions were given about visualization and visual images to make clear this process for mathematics learning and future academic researches.