Concepts such as teachers' professional development and students' achievement act as the driving force for the development of each in a causal relationship in EFL teaching, as in many other disciplines. The purpose of this study is to investigate the change Understanding by Design (UbD) made on teachers' professional development and students' achievement. The study was carried out through action research design. The participants were 10 teachers and 436 students. Within action research based UbD studies, the instructors received training, formed four groups and made three designs and implemented them. As qualitative data collection tools, unit designs and one-to-one interviews were used. The quantitative data of the study were collected through English achievement scores. We instrumented content analysis for the qualitative data and the analysis of covariance and independent samples t-Test for the quantitative data. The findings indicated that action research based UbD studies had positive contributions to teachers' professional development process and students' English achievement.