A fundamental role of education is to equip students with sound justificatory reasoning capacities to support one's beliefs or actions and to guide rational decisions about complex problems. However, empirical research into justificatory reasoning among students is virtually non-existent and this impedes education research. This study illustrated the measurement of justificatory reasoning ability and investigated how tertiary students might use different justificatory reasoning forms (absolutism, relativism and evaluativism) depending on the reasoned domain, a controversial domain versus a moral domain. It further tested how different justificatory approaches might relate to varying willingness to engage in argumentation, as well as how they might engender beliefs with different strength of convictions. The results suggest that absolutism was preferred for moral domain, whereas relativism was preferred for controversial domain. However, those who used evaluativism were most willing to engage in discourse to defend their beliefs, and their engendered beliefs were also the strongest. Besides demonstrating how justificatory reasoning may be measured quantitively, the study provides guidance to educators on designing curriculum to develop justificatory reasoning ability. The findings also suggest that educators should not unconditionally focus on developing evaluativism in students, supposedly the highest form of justificatory reasoning.