This research aims to describe the decision-making of pre-service teacher in designing lesson plans and their implementation in learning mathematics. This type of research is qualitative research with a descriptive approach. Determination of subjects in this research using a purposive sampling technique. The subjects in this research are students of the Mathematics Education Study Program at one of the universities in East Java, Indonesia. The subjects is the winner of a microteaching competition at the university level. Learning activities are observed in the preliminary, core, and closing activities. Because this is qualitative study, the main instrument is the researcher and is assisted by observation sheets, video recordings, and interview guidelines. The conclusion is that subjects make decisions at each step of learning with the stages of building ideas, clarifying ideas, and assessing the fairness of ideas. At the idea-building stage, subjects build ideas with several/various ideas for activities to be carried out and adapted to the conditions and time of learning.