Designers depend on representations to externalize their design thoughts. External representations are usually in the form of sketches (referred to as traditional media) in architectural design during the conceptual design. There are also attempts to integrate the use of digital representations into the conceptual design in order to construct a digital design medium. This thesis aims at gaining an insight on designersi cognitive processes while sketching in digital versus traditional media. The analysis of cognitive processes of designers based on their protocols is necessary to reveal their design behaviour in both media. An experiment was designed employing six interior architects (at Bilkent University) solving an interior space planning problem by changing the design media they work with. In order to encode the design behaviour, a coding scheme was utilized so that inspecting both the design activity and the responses to media transition was possible in terms of primitive cognitive actions of designers. The analyses of the coding scheme constituents, which are namely segmentation and cognitive action categories enabled a comparative study demonstrating the effect of the use of different media in conceptual design phase. The results depicted that traditional media had advantages over the digital media such as supporting perception of visual-spatial features, and organizational relations of the design, production of alternative solutions and better conception of the design problem. These results also emerged implications for the computer aid in architectural design to support the conceptual phase of the design process.