Design-based learning (DBL), a pedagogical approach to integrating science, technology, engineering, and mathematics (STEM) education, has been internationally promoted in K-12 curricula. Despite DBL's variations, research has indicated it can facilitate students' content learning. However, it is less clear whether DBL can foster students' STEM identity. The lack of research on this issue is critical, because STEM identity is a factor influencing secondary students to pursue STEM-related careers. This study aims to examine the influences of DBL on two types of learning outcomes achieved by eighth grade students. Two classes of 32 students, denoted Class 1 and Class 2, engaged in different approaches to DBL: respectively, design-through-science and science-through-design. Data were collected before and after DBL through using a conceptual test and a self-reported questionnaire measuring STEM identity. The data were descriptively and inferentially analysed to compare each class's learning outcomes and the differences between these classes. Results reveal that, compared to Class 1, Class 2 more effectively improved their content learning and demonstrated more aspects of STEM identity. However, promoting content knowledge and developing STEM identity over a multi-week duration is challenging. It is necessary to integrate a specific framework into DBL to cultivate content learning and STEM identity.