This study explored the influence of proof understanding strategies and negative self-concept on undergraduate Afghan students' achievement in modern algebra 1. To examine the relationships among proof understanding strategies, negative self-concept and achievement in modern algebra 1, we used structural equation modeling on data collected from three classes of students taking modern algebra in two consecutive years of 2016 and 2017. Participants of this study included 139 sophomore Afghan students, 40.29% male and 59.71% female. Conducting SEM analysis resulted the following findings: firstly, negative self-concept had significant negative influence over students' achievement in modern algebra 1; secondly, proof understanding strategies had significant positive influence over students' achievement in modern algebra 1; thirdly, negative self-concept had significant negative influence over proof understanding strategies; finally, the data fitted very well the SEM model, implying that proof understanding strategies and negative self-concept significantly influence students' achievement in modern algebra 1.