STEM education emphasizes improving student learning by linking abstract knowledge with real-world problems and engaging students in authentic projects to solve real-world problems. Accordingly, project-based learning has been widely promoted in STEM programs and has shown a promising impact on student learning. However, solving real-world problems in STEM projects involves complex processes. It remains unclear how students engage in complex problem-solving processes in STEM projects and how their processes may differ among students. This study was conducted with secondary school students who engaged in a design-based STEM project in small groups. The findings show that questioning and responding appeared most frequently and connected with other elements in group discourse, while argumentation and justification appeared least frequently. The findings reveal distinctive discourse patterns that differ among high-, medium- and low-performance groups, based on which the implications of the findings were discussed.