Contribution: This paper qualitative research delineates the thinking and behavioral patterns that reflect students' advanced level of epistemological thinking in project-based learning (PBL) activities and the associated impact factors. Background: PBL has been widely adopted in engineering education for its effectiveness in improving students' problem-solving abilities, collaboration skills, and academic achievement. The four stages of Perry's theory--Dualism, Multiplicity, Relativism, and Commitment (within Relativism)--depict students' epistemological development from dualistic to contextual relativistic thinking. Prior research suggested that participation in PBL could benefit students' development to relativistic thinking--the most revolutionary change in Perry's model. Nevertheless, how epistemological thinking was exhibited in the context of PBL, and the relationship between students' epistemological thinking and PBL learning activities both remain unclear. Research Questions: 1) What are demonstrations of students' relativistic thinking in PBL activities? and 2) What are the factors in regard to students' relativistic thinking in PBL activities? Methodology: Built upon quantitative results obtained using a modified Zhang's cognitive development inventory (ZCDI) developed in the context of Perry's theory, this paper explored demonstrations of students' relativistic thinking in PBL in a qualitative manner, and analyzed the associated factors based upon one-on-one interviews with 17 engineering students. Findings: In PBL activities, students' epistemological thinking is reflected in their solving problems within constraints, conducting feasibility analyses, exhibiting commercial awareness, having their thinking broadened, and more. Their epistemological thinking can be impacted by factors, such as guidance from professors, collaborations with peers, communications with other stakeholders, and complexity level of projects.