Political legitimacy is the sine qua non of governing in the contemporary world. In the absence of legitimacy, brute force alone will be the sole means of sustaining a government. Given the large size of the youth in the population structure of Iran, measuring legitimacy in the attitudes of the members of the third generation of the Revolution gains particular importance. In this study, based on a secondary analysis of the data collected in four national surveys, we attempted to measure the changes in youth attitudes towards legitimacy of the political system. Items in the instruments of the surveys that measured legitimacy were drawn out and made commensurable before being used to illustrate the trend that existed from 2001 to 2011. To better tap all aspects of the concept "legitimacy," we divided it into its two essential components, namely "legality" and "justifiability," and then drew upon items that represented these categories in the surveys on four different times during the study period. In this vein, the changes in youth attitudes toward legitimacy of the political system were identified. Study results indicate that in the first half of the period under study, a rising trend is observed. However, in he second half of the period, perceptions of legitimacy seem to show a decreasing trend. At the end, the more salient factors responsible for the drop in legitimacy are identified.