Background/purpose. The role of early childhood education (ECE) teachers is very important in building the foundation of mathematics, such as recognizing counting, patterns, and problem solving. However, many teachers still face challenges in understanding mathematical concepts in depth. This study aims to investigate the level of Mathematical Knowledge for Teaching (MKT) among early childhood education teachers and understand the factors influencing their MKT levels, serving as a foundation for designing an effective professional development programmed (PDP). Materials/methods. This study uses an exploratory case study design because it allows researchers to understand the complexity of a problem and explore it in depth. This study involved 5 ECE teachers in West Java, Indonesia, with the following criteria: more than 10 years of teaching experience, BA in ECE, teaching in a public ECE school, and having more than 20 students per class. The data collection techniques include participant observation, tests, interviews, and documentation. The data analysis technique uses thematic analysis. Results. Research findings show that the limited knowledge of MKT of ECE teachers, especially related to basic mathematics skills and children's mathematics development, is influenced by several factors. These factors include the lack of special training in mathematics teaching, limited appropriate learning resources, limited teaching experience, and a lack of focus on professional development in mathematics. Conclusion. Improving ECE teachers' MKT requires targeted training, better resources, and focused professional development. The contribution of this research is that it can improve the quality of mathematics teaching in ECE and support children's mathematics achievements in the future.