This paper reports a two-year research project studying Indonesian primary and secondary teachers' mathematics-related beliefs (MrB) and mathematical problem-solving knowledge for teaching (MPSKT). In the first year, a quantitative study involving 80 primary teachers, 70 lower secondary school, and 55 upper secondary schools from four districts in East Java province, Indonesia was carried out to examine whether there is a difference between the MrB and the MPSKT among them. In the second year, a multiple case study involving ten primary teachers and 13 lower secondary teachers was conducted to understand their MrB and MPSKT through some particular issues related to three domains of MrB: nature of mathematics, mathematics teaching, mathematics learning. Results indicate that there is no significant difference of MrB between primary and secondary teachers, while there is a significant difference between the MPSKT of primary teachers and secondary teacher. Findings also suggest that inconsistencies not only occur between the three domains of MrB but also occurs between particular issues discussed within one domain. Also, this study highlights that teacher beliefs about nature of mathematics were mainly influenced by teachers' experience when learning mathematics during their schooling experience while teacher beliefs about teaching and learning were mainly influenced by the demand of current reform of mathematics curriculum.