The purpose of this study is to examine the development of youths\' career maturity over time empirically. For this, 425 male and female students of middle and high schools in J city were selected. The participants had taken Career Maturity Inventory by Krivet twice. They took the second test 8 months after the first test. The results of cross-sectional analysis showed inconsistent career maturity level over age. These results did not support the hypothesis of career development theorists. But the results of longitudinal analysis showed considerably consistent career maturity level over age. These results supported the hypothesis of career development theorists. The developmental tendency of youths\'s career maturity was different according to youths\' sex or age. Middle school students\' career maturity was developed positively in ability and career preparation behavior domain to the contrary negative change in attitude domain. High school students showed the contrary development in each domain.