Each level on van Hiele's model has its characteristics, such as terminology, concept, and reasoning strategies. This study attempts to reveal epistemological obstacles on all van Hiele's levels of geometric thinking in the topic of geometric transformation. Thus, a student could overcome the epistemological obstacles in transformation geometry and develop appropriate reasoning strategies. That way, van Hiele's geometric thinking could promote level by level. This research is a case study that investigates and analyzes the epistemological obstacles on the topic of transformation geometry for each level of thinking of the van Hiele model of geometry. The study was conducted on ten prospective mathematics teachers who had received lectures in transformation geometry at a university in Indonesia. The results found in this study indicated epistemological obstacles at each level of geometric thinking in van Hiele's model.