The objective of this research is to understand how a learning trajectory supports pre-service primary teachers in their noticing of students' mathematical understanding. A total of 95 pre-service primary school teachers used a learning trajectory related to the part-whole meaning of the fraction concept to interpret students' understanding and provide instructional decisions. The findings indicate that the learning trajectory provided pre-service teachers with a specific language to describe students' understanding and helped them to notice students' mathematical understanding. Furthermore, the pre-service primary teachers who produced a more detailed discourse also proposed more suitable activities based on students' understanding. These findings suggest that the learning trajectory can act as a scaffold to notice students' mathematical understanding.