This study is due to investigate five eleventh graders of how they developing conjecturing and justifying power in the context of binary number pattern-finding problems. The framework of the research is situated in the social perspective which considers that the development of students’ conjecturing and justifying power should be much relied on interactions, dialogues and argumentations within peers. Qualitative case study design and analysis strategy is adopted in the study for constructing a holistic viewpoint of students’ mathematical conjecturing and justifying behavior. Research results show that students’ inherent capabilities could be revealed when they are engaged in an atmosphere of conjecturing. Besides, students could not only apply specializing and generalizing strategies flexibly but also propose impressive justifications to make others convinced during the process of conjecturing. All of the findings seem to imply that relational understanding is crucial for students to develop their conjecturing and justifying power.