The process of knowledge transformation is one of the cognitive processes that occur in learning. But in reality it is still rare that teachers pay attention to the process of transforming students' knowledge in learning, so that teachers do not know the process of transformation done by students in learning. Therefore, in this research, it aims to describe the process of transformation of Mathematics knowledge of elementary school students in classroom learning. The process of knowledge transformation in this research consists of five stages namely, socialization, connection, representation, justification, and internalization. This type of research approach is qualitative research. The subject of this research is elementary school students in Tulungagung city Instruments in this study are the main instruments and auxiliary instruments. The main instrument is the researcher himself, while the auxiliary instruments are: Tasks Math and interview guides. The results of this study at the stage of socialization of the subject of sharing knowledge about the problem solving plan, At the connection of the subject connect the mathematical concepts with images and connect the mathematical concepts with the environment, At the subject representation stage create an image to illustrate the shape of the pool, At the justification stage of the subject answered correctly along with the reasons gradually, and at the stage of internalization of the subject is able to explain the concepts and procedures on other situations, Recommended for classroom learning, teachers can design learning that takes into account the five stages of the knowledge transformation process.