Abstract: Autonomous map building of mobile robots is a fundamental task in robotics research. This paper focuses on the problem of uncertainty sensor information processing in map building. In order to process sonar information and build grid-based map of environment, a new method based on grey system theory Is proposed which interprets and combines sensor information. We introduce the concept of "Grey Number" to model and handle the uncertainty of sonar sensing information. Furthermore, a novel fusion method is proposed, which refers to the paraphrase of information by grey system theory. Map building experiments are performed both on simulation platforms and real mobile robots. Experiment results show that our method is robust and accurate.