The study aims to explore how pre-service mathematics teachers reveal their knowledge of infinity during the lesson planning process in the context of limit. Specifically, we adopted the dimensions of mathematics teachers' specialized knowledge, which are related to mathematical knowledge and its relation with teaching. We conducted an exploratory case study design in a two-cycle lesson planning process with three senior pre-service mathematics teachers. The findings indicated that in the first planning process the participants did not use infinity as a way of thinking, rather they focused on paradoxes and potential infinity. After they taught their lesson plan, they started to think about the complexity of infinity not only in the context of limit but also in the concept itself during the lesson planning process. The findings did not cover the knowledge of infinity; rather, they yield important implications for lesson planning process to reveal teachers' knowledge for teaching.