This study aims to investigate ethical tensions involved in mathematics teachers' instructional decisions. The research was designed as a multiple case study and three mathematics teachers were chosen as cases with criterion sampling strategy. Data were collected through a series of in-depth interviews via semi-structured questions and scenarios. Moral distress framework, first proposed in nursing research, was employed to analyse the data. Our analysis suggested that teacher decisions were shaped on the basis of scientific/technical as well as ethical considerations. Findings pointed to the operation of three different types of ethical tensions in the production of mathematics teachers' particular instructional decisions: moral distress, moral dilemma and moral uncertainty. We proposed that ability to perform desired actions and to provide rationalizations are among the key discriminants for different forms of tensions. We discussed the features of different types of ethical tensions and shared the implications about the development of moral agency as well as the consideration of ethics in mathematics teacher education.