Aim of this study was to investigate a secondary mathematics teacher's beliefs about the nature of mathematics and to see how these beliefs were manifested in his teaching practice. I conducted a life history research to explore the biographical learning processes of a male teacher, which occurred due to transition in his work situation. It highlighted the key stages in his academic and professional life, and illustrated the critical points, which reshaped his views about mathematics, and its teaching and learning. The data were collected through interviews, classroom observations, informal discussions and documentary evidences. In order to investigate the beliefs of a teacher about the nature of mathematics, the life history approach is an appropriate method, as Johnson and Golombek (2002) state that narrative inquiry enables teachers to organize and articulate, what they know, and believe about the subject, and its teaching and learning, and to communicate them to the researcher. They further argue that their stories reveal the knowledge, ideas, perspectives, understandings, and experiences that guide the researcher in understanding their beliefs. The findings of the study showed that the teacher considered mathematics as a subject, which was practical, a language of communication, a sequence of topics, a process of thinking, and problem solving. Findings also showed that beliefs are not static; rather, they change when new knowledge and desirable experiences are gained. Beliefs have a deep influence on practices, and practices form the beliefs, thus these two are interrelated phenomena. Findings suggest that it is necessary to study teachers' beliefs about the nature of the subject, in order to bring change in their practice, because, without knowing teachers' beliefs, their practices cannot be changed. Therefore, I conclude with the suggestion that in teacher professional development programs a study of the nature of mathematics teachers' beliefs may be included.