Mathematics self- efficacy is generally defined as individuals' beliefs or perceptions regarding their mathematical abilities. The aim of this quantitative study was to conduct a validity analysis of a self- efficacy instrument toward mathematics reasoning among 184 undergraduate public university students. Students clearly distinguished between their self-efficacy in doing and understanding mathematics reasoning and their self-efficacy in accomplishing assignments in their university mathematics lesson. In these quantitative studies, self-efficacy which demonstrated convergent and discriminant validity was performed, which corresponded to the Bandura theory and included three dimensions: course self-efficacy, exam self-efficacy, and future self-efficacy. The research design utilized casual correlation technique to investigate the effect of self-efficacy on mathematics reasoning. The reliability analysis of Cronbach Alpha internal consistency reliability revealed that self-efficacy instrument that was constructed was extremely reliable and can be utilized by researchers to measure university students' self-efficacy toward mathematical reasoning.