This study analyzed gender differences in variability with the results of National Assessment of Educational Achievement on Mathematics from 2003 to 2006, which subjects are 3rd, 9th and 10th grades. The purpose of the analysis was to explain gender differences in mean of mathematical ability scores from a different angle. The results and conclusions obtained in this study are as follows: First, regardless of grades and mean differences, male assessment scores were consistently more variable than female assessment scores. Second, for same male and female students, scores in later grades were almost consistently more variable than those in earlier grades. Third, for all grades, the ratios of males in an advanced and below-basic levels were generally higher than those of females in the same achievement levels.