This research is conducted among 65 seventh graders (12-14 years old) who attend introductory course on physics. Tests and interviews are used to trace the roots of the students' misconceptions about mass. Results from the research reveal serious weaknesses in students' understanding of concept of mass, and its confusion with concepts of density and weight. Pre-conceptions about dependence of volume and mass of physical body and concept of heavy materials have deep impact on students understanding of mass and its measurement. Also, poor understanding of inertia signals that it might be more natural way for students to present mass as property of matter that comprises the body. Sources of misconceptions are found in the prior students' experience in informal, as well as in formal education. Later include students' vocabulary development, physics curriculum structure, and physics textbook. Weaknesses are detected in the non-physics teachers' competences who teach physics concepts. All of the research findings are supported by similar findings in other researches, which confirm that detected misconceptions are generic. In order to minimize the existence of detected misconceptions, directions for revision of the curriculum and textbook are proposed. Also, directions for methodological approach are given.