Background In the product design field, different stakeholders communicate with each other with design constraints. This paper reports on how designers use different constraints in the sketch phase. Methods Senior students majoring in industrial design participated in the design experiment. Their idea sketches and interview data were collected, which were then categorized with a coding scheme developed from Lawson"s design problem model. Results There are three major findings. First, unlike architects, product designers consider not only "designer constraints", but also other generators" constraints in the sketch phase. Second, the constraints related to the interaction between users and objects mostly fall into external constraints. This reflects the fact that most user interactions with products occur on the surface. Lastly, fewer constraints in client and legislator categories are adopted while a large number of constraints in the user category are used by designers. Conclusion Based on the findings, we argue that if Lawson"s model is applicable to the product design field as well as architecture, it needs to be modified or further developed by considering the characteristics of product design that are different from those of architecture.