This study investigates the development of prospective secondary mathematics teachers' (PSMTs') noticing skills in a course conducted at the last year of their teacher education program. PSMTs watched and analysed different instructional videos, including an unknown teacher's, their mentor teacher's, and their own teaching videos. The data were collected from five PSMTs through reflection papers and group discussions about what and how they noticed in the videos. The results showed that the PSMTs had different levels of noticing at the beginning of the course and developed their noticing skills over time through the research. It was found that PSMTs' noticing skills differed across different video types. This study revealed the importance of experience in real classroom environment for the development of PSMTs' noticing skills. Based on the results reached in the research, some suggestions were included.