Mathematics anxiety (MA) refers to negative cognition and negative affect that interfere with mathematics performance. We examined the dimensionality of MA, measurement invariance across males and females, and whether the strength of relations between MA and mathematics performance varies by dimension and gender among 245 sixth-grade students. Students were assessed on MA, arithmetic fluency, and on-grade-level computation skills. The confirmatory factor analysis and measurement invariance tests indicated that two distinct dimensions, cognition and affect, best represent MA across both genders. For the full sample, on-grade-level computation skills showed a significant negative relation with the cognitive dimension only, whereas arithmetic fluency was not significantly related to either dimension. We did not find any significant gender differences. Our findings provide support for the cognitive and affective model of MA and negative impact of MA on mathematics performance. Our findings further suggest incorporating the cognitive component in identifying and alleviating MA, and improving mathematics outcomes. [For the corresponding grantee submission, see ED629294.]