With this study we aimed to evaluate the effectiveness of a collaborative and reflective school-based training model on the teachers' professional transformation in a middle school in Turkey. This study emerged as a result of a need for this school to transform its instructional approach to the understanding by design (UbD) approach. Considering the needs of teachers for instructional design, a 1-year training programme was planned. Thirteen teachers from different subject fields in the school participated in this case study. The teachers prepared 4 UbD plans collaboratively under the coaching of the researchers, implemented them in the class, and participated in the reflection meetings after each implementation. While evaluation of plans showed their progress, focus-group interviews revealed that this training had changed the teachers' instructional approach and developed their design skills. Additionally, even though there were some challenging issues such as intense workloads, time constraints, coordination problems, the teachers generally emphasised the contributions of studying UbD to their peer collaboration, classroom practices, and their students' understanding and learning. We concluded that not only this collaborative and reflective training model but also the UbD itself had positive effects on teachers' professional transformation.