In this study, middle school students' (8th grade, n = 24) problem-solving processes were investigated while implementing a design-based science learning (DBSL) approach. DBSL tries to incorporate science learning with the processes of engineering design. A DBSL module was developed by the research team within which students were expected to design an ice cream making device from simple and easily available materials. The goals of the study were: (a) To develop an understanding of the processes of student design including difficulties they face within the DBSL setting; (b) to determine how science knowledge was used by students in a design situation; and (c) to explore how student design processes and design products can be characterized and eventually, assessed. Data were gathered from students' written reports, video-recorded classroom observations, and teachers' oral feedback. The findings reveal that the crucial aspects for design success were the students' understanding of the scientific phenomena, the operational principles behind the ice cream making device, and the understanding of the design criteria. Lack of one or more led to unrealistic design ideas. The initial difficulties were overcome by peer support, teacher guidance, and trial and error experiences. A set of assessment criteria, able to characterize student design products, were developed. As a result of this study, practical guidelines for curriculum developers and teachers on how to facilitate further implementation of DBSL in the classroom are provided.