This study aims to determine pre-service teachers' conceptual understanding of black-body radiation. Designed as a qualitative study, it employed a case study design, which is one of the descriptive research methods. The study group consists of 18 pre-service physics teachers enrolled as seniors. An opinion form and a concept map were used as data collection tools. In other words, results were intended to be comparatively examined by using two different qualitative data collection methods. Pre-service teachers' answers were analysed with the help of answer sculptures. At the end of the study, it was determined that while pre-service teachers generally have the correct conceptual structure, they still have some specific gaps in knowledge. It was seen that pre-service teachers have problems with understanding, especially the Wien law and electromagnetic radiation taking place in black-body radiation.