This qualitative case study investigated the improvement of two beginning middle school mathematics teachers' instruction to implement cognitively demanding tasks and to orchestrate productive discussions by using five practices framework. For this purpose, the study was carried out in two phases: pre and during the professional development (PD). Data were collected through classroom observations, student artifacts, interviews, and teachers' planning and monitoring documents. The results displayed that, before the professional development, teachers did not make plans based on student thinking, not provide adequate time for students to explore the tasks, not build an environment based on classroom discussions, and they usually implemented cognitively low-demanding tasks. Along with the professional development, teachers deeply considered on the purpose of cognitively demanding tasks, but didn't reach an expected level on detailed anticipating. They constructed a classroom setting based on students' exploration of tasks and consideration of multiple solutions. They purposefully selected and sequenced different solutions, initiate discussions to connect students' approaches and underlying concepts. However, they didn't reach the expected level of making connections among different solutions. Eventually, they mostly maintained the cognitive demand of high-level tasks in the professional development period.