Modul Ajar, the Merdeka Curriculum-based lesson plan, is the current issue in the Indonesian context after the application of the Merdeka Curriculum. New challenges and adaptations are required, especially by pre-service EFL teachers. Meanwhile, few studies explored the reflective practice focused on lesson planning. This research aimed to: 1) explore the factors that influence pre-service EFL teachers’ reflection on lesson planning during their teaching practicum; and 2) explore the impacts of the pre-service EFL teachers' reflective process on their lesson planning designed based on the Merdeka Curriculum. Qualitative methodology with a multiple-case study design was employed, and purposive sampling was used to find two participants in the high school context. Next, the documents Modul Ajar created by participants over time were analyzed, followed by semi-structured interviews. Secondary data was observation and documentation. Results indicated six factors that influenced the reflections: 1) environment, 2) competency, 3) behavior, 4) belief, 5) identity, and 6) mission. The most emphasized factors were environment and competency. Belief was also emphasized in one of the participants’ reflections. The impact of the environment can affect the competency of pre-service EFL teachers in lesson planning. Meanwhile, belief can lead to incompetence in understanding the components of Modul Ajar. The finding implies that providing a supportive environment can lead to the best performance of pre-service EFL teachers. As reflection-on-action is important to enhance their awareness and teaching process in the future, this research suggests using reflection-on-action focuses on lesson planning to provide another context, for example, elementary school education.