Assessments of student learning outcomes (SLO) have been widely used in higher education for accreditation, accountability, and strategic planning purposes. Although important to institutions, the assessment results typically bear no consequence for individual students. It is important to clarify the relationship between motivation and test performance and identify practical strategies to boost students' motivation in test taking. This study designed an experiment to examine the effectiveness of a motivational instruction. The instruction increased examinees' self-reported test-taking motivation by 0.89 standard deviations ("SDs") and test scores by 0.63 "SDs." Students receiving the instruction spent an average of 14 more seconds on an item than students in the control group. Score difference between experimental and control groups narrowed to 0.23 "SDs" after unmotivated students identified by low response time were removed from the analyses. The findings provide important implications for higher education institutions which administer SLO assessments in a low-stakes setting.