The purpose of this study is to investigate how prospective mathematics teachers' develop their knowledge of mathematics lesson plan in a Teaching methods course. We conducted this qualitative study with eight prospective secondary mathematics teachers studying at at a state university in Turkey. We have collected during the course of "Teaching methods in mathematics education". We have asked questions about planning mathematics lessons to 60 prospective teachers at the beginning of the course. We have individually interviewed with eight prospective teachers. The prospective teachers prepared lesson plans in pairs based on the acquisition in the curriculum that was provided by the instructor of the course. After the Teaching methods course, we again interviewed these prospective teachers with their peers whom they prepared their lesson plans with. In beginning of the course, some of the prospective mathematics teachers confused mathematics lessons plan with mathematics curriculum. We have explored two categories at the beginning of the course: knowledge of components in lesson plans and knowledge of students' motivation and understandings. The knowledge of teachers' self-assessment is the other category, which evolved from the data at the end of the course. We have found that prospective secondary mathematics teachers' knowledge of mathematics lesson plans change from structural level to instructional level. In the beginning of the course, the prospective teachers mentioned sections of a lesson plan, elements of teaching process, and classroom context. After the course, they gave detailed information about how to organize an instructional process considering students' motivation and understandings. The prospective teachers also put the acquisitions in center of the lesson plan. They had never referred mathematical content before they took the course. However they realized the importance of their own mathematical content knowledge while designing a plan.