Learning and teaching function transformation has a significant place since it provides new opportunities for learners to use, reflect and discover knowledge related to the concept of function. This study's main goals related to the importance of learning function transformation are to examine pre-service mathematics teachers' (PMTs) mental constructions of function transformation from APOS theory perspective and identify their concept images and concept definitions of function transformations. Two frameworks, APOS theory and concept images and concept definitions, were considered in this study. The qualitative study adopted case study design. The participants were three female PMTs. Clinical interview methodology was used to collect the main data. Data were analyzed based on the APOS framework, and the descriptive analysis method was used in the process. The results of the study present PMTs' mental constructions as well as to their concept images or definitions of function transformation. In this study it is found that PMTs had limited knowledge regarding function transformation, but their mental constructions were changed throughout the process and they partially overcame their difficulties related to the families of functions and related function concept images and definitions.