This study examined how the levels-of-sophistication framework supports pre-service mathematics teachers' professional noticing of students' reasoning about length measurement. Three pre-service teachers were asked to analyse students' written solutions in the tasks that reflected different characteristics of students' reasoning before and after participating in an intervention based on the "Levels of Sophistication in Students' Reasoning about Length" (Battista, 2006) conceptual framework The findings indicated that the levels-of-sophistication framework enabled the pre-service teachers to give their full attention to students' mathematical understanding and to provide proper instruction to support students' learning and hence, it had a significant role in the improvement of the professional noticing skills of these pre-service teachers. Thus, it is suggested that the levels-of-sophistication framework can be used to support pre-service teachers' professional noticing skills and to better prepare them to teach length measurement based on students' reasoning.