The purpose of this study is to investigate how prospective elementary teachers understand the concepts of mass and weight. The Korean elementary science curriculum provided the achievement criteria into the unit of weight to teach the concept of weight but did not teach the mass concept. Therefore, compared with other countries, the Korean science curriculum may imply difficulty in understanding the concept of weight without being able to distinguish the concept of mass. The 79 prospective elementary teachers enrolled in 3 departments of the one national university of education were selected for convenience as the research subject. The questionnaires asking for understanding of weight and mass were developed, and the responses were analyzed to investigate schematically whether the these two concepts were distinguished. Based on this result, we discussed the problem of teaching the weight concept without understanding the mass concept, and summarized ideas on how to understand and teach the weight by basing it on the mass concept. This research proposed the method to promote a comprehensive understanding of weight and mass in the elementary teacher curriculum.