In this paper, we examine the past and future changes of mortality which may emerge as another momentum and its effect on the population change in Iran. The research question is whether the population can be stabilized only by the stability of fertility rates at or around the replacement level. The question has been derived from a theoretical demographic problem which has become a socio-political discourse in recent years. Many studies have suggested that mortality patterns, particularly adult mortality, can also create a momentum like the general population momentum which can affect the population growth of Iran in the future. Using stable population theory and data from censuses of the country, this paper attempts to address such demographic consequences of the mortality changes and its effects on the age structure and increase in the size of the population. Results show that increases in cross-sectional life expectancy at birth have improved survivorship for younger cohorts in the early stages of the mortality transition and for older cohorts in its later stages. Therefore, this effect reveals that the country has been experiencing the momentum of mortality changes and individual aging through which the aged population increases faster than younger cohorts. The differences have been approved by the results of the 2011 census.