The aim of this study is to reveal relations between creativity in design and cognitive development levels of designers with a Post-Piagetian approach. In this study design processes were deeply inspected with the aim of obtaining more comprehensive information about the effects of individual’s cognitive status on the design processes, problem-solving and idea generation quality. In this field, studies done so far have been conducted on test results as indicators of creativity. In this study, however, protocol analyses were also evaluated in addition to testing the results, and thanks to this addition, it was possible to determine the qualitative and quantitative effects of the cognitive development levels on creativity. Total twenty-five students voluntarily participated in the research. To collect data, two different methods were employed. Social Paradigm Belief Inventory (SPBI), was employed to determine students' cognitive stages and scores, and Think-aloud protocol, was used to elicit verbal reports of thought sequences of students. In order to analyse verbal reports for investigating designers' cognitive activities, Linkography method was conducted and general statistics of segments were measured. To exposure, the relations between creativity in design and cognitive development levels of designers, SPBI scores and obtained general statistical values of design sessions were correlated and findings showed that there is a strong and significant correlation between cognitive development levels and creativity in favour of dialectical thinking. Addition to this, participants’ partial protocols were analysed syntactically, and the networks were inspected to reveal the impacts of postformal thinking on creative design process with the help of Function-Behaviour-Structure (FBS) Ontology. The findings of the syntactic analysis also supported the finding of the correlations.