The goal of this study was to investigate understanding of in-service elementary school teachers in Taiwan about number sense, teaching strategies of number sense and the development of number sense of students. Data were gathered through interviews of nine elementary mathematics teachers, regarding their understanding about number sense. The data indicated the followings: (1) The intuitive of number sense of teachers described as good intuition or sensitive about number sense. They stress on understanding number meanings and ignore these relationships about quantity and operation; (2) The teachers have similar views about teaching strategies of number sense; (3) The teachers believe that children with good number sense have well-understood number meanings and relationships, awareness that multiple strategies exist, and recognize reasonableness of data and calculation; (4) The teachers lacked for knowledge of number sense, so that they cannot address the complete teaching strategies of number sense; and (5) The background variables of teachers are related to the understanding of number sense.