The age structure is one of the dynamic aspects of demographic changes which have very strong interrelation with main items of natural growth rate, including birth rate and death rate. These two factors determine age structure on the one hand and mutually can be influenced by the age structure on the other hand. One feature of this relationship, appears in the theorem of "momentum", which first was introduced by Keyfitz and further discussion is proposed by Preston. The momentum of population growth resulting from the childbearing of last cohorts before the drop in fertility and subsequently the future childbearing will be influenced by the decline to replacement level of fertility. This paper examines the consequence of fertility decline in Iran. This study used data from 1976 to 2006 decennial censuses and the projection data file (variant medium) in world population prospects (revision 2006). The result shows that in the scenario of a sudden drop in fertility to replacement level, as Preston (1986) argued, the population segment under age T, the mean length of generation, remained growth free and the size of population mainly increased in ages T and over. Therefore, contrary to our presumption, the most important consequence of the momentum is the process of population aging. As a result, further decline in fertility only will increase the pace of the process.