This study examines the role of motivation in the academic success of ethnically diverse STEM disciplines. Grounded in Expectancy-Value Theory (EVT), this research study explores the relationship between effort cost (the energy and effort required for an academic task) and attainment value (academic identity) predicting GPA and academic self-efficacy. Longitudinal data from 266 participants at two Minority Serving Institutions (MSIs) revealed that effort cost negatively predicted GPA and self-efficacy, while attainment value yielded modest results in positively predicting academic self-efficacy. Although attainment value did not significantly moderate these effects, findings highlight the importance of accounting for attainment value and effort cost with respect to GPA and academic self-efficacy over time. While motivation has been researched extensively, less is known about how these processes unfold for diverse students in STEM disciplines over time. This research underscores the importance of understanding motivational patterns in diverse populations to inform support strategies. By examining how attainment value may mitigate the negative impact of effort cost, the study contributes to enhancing academic outcomes and supporting the success of diverse students in rigorous STEM disciplines.