Teachers´ professional development is crucial to bring about educational effectiveness and school improvement. In the New Century teachers must be able to work effectively within context of change, paradox and instability. However, the current policy approaches to teacher education in Sweden are far from this demand of developing teachers as critical professional experts. In this article, we exam the use of the ideas of action research in carrying out a course within an in-service teacher training program for master degree at Mid Sweden University, in which teachers� inquiry into their own practice and their reflective learning were stressed. Fourteen teachers who took this course participated in this study. The data we collected into this study includes questionnaires, the writing reports of teachers� action research, the curriculum documents of programme, and conversations with the focus group. Using the constructive frameworks of professional knowledge, teacher professionalism, action research, reflective practice in analysing the data, we find that teachers become more aware and sensitive to questions and ideas concerning educational and pedagogical issues, as well as develop knowledge and a heightened sense of their own professional work through this kind of reflective practice. We argue that carrying out in-service training programmes based on ideas of action research can make teacher education more relevant to the demands for teachers� learning and their professional development, support the tendency of an inclusive approach in schools, and thus promote educational effectiveness and school improvement.