The aim of this study is to investigate both the planning and implementation process of a modelling task by a middle school mathematics teacher. The participant of the study, which was conducted as a single case study, is a middle school mathematics teacher who have knowledge and experience in modelling. The data collecting tools of this study were the lesson plans of the teacher, the observation notes taken by the researchers during the implementation of the modelling task, the video records of the implementation, students’ solution papers and the voice records of the pre- and final-interview conducted with the teacher. Whole data were analyzed according to a framework for modelling implementations developed by the authors through descriptive analysis. The teacher planned his in-class implementation to consolidate the circle concept and revised an existing modelling task by developing appropriate materials for the task. He asked students to arrange their desks appropriate to group work, decided to enable students to evaluate both the implementation and their peers, and announced their students that they would present their group solution and evaluate each other at the end of the lesson. During the implementation, he first reminded prior knowledge about circle and then presented the materials. He shared instructions about group work in the context of sharing social norms and gave information about the peer assessment form, which the students filled at the end of the lesson. During the students were solving the task, he intervened about enabling self-assessment in the groups, considering realistic values and comparing mathematical results with the problem context. In addition, during the solution, he tried to establish socio-mathematical norms by making explanations. After the solution process ended, student groups presented their solution approaches and discussed different solutions by comparing each solutions. Peer assessment form and affective evaluation form regarding the implementation were filled by the students at the end of the implementation. This study contributed the literature regarding modelling implementations and presented a different view about teacher actions in planning and implementing a modelling task.