The second demographic transition id basically a theory of "low fertility" in the late modern societies. According to this theory, low fertility level and its duration relate to changes at the societal level and their associated changes in partnership and reproductive behavior that occurred in those societies in the last decades of the twentieth century. The second demographic transition theory fits well in Scandinavia and relatively well in other countries in Northern Europe and English-speaking societies outside Europe. But, as we move from Northern Europe to the south and from Western Europe to the east and in advanced countries of East Asia, plausibility of the theory in some respects declines. Therefore, those qualifications should be accounted for, when we use this theory to explain the low fertility level in Iran. Furthermore, Iran should be considered as a peripheral society in which changes are partly dependent and unbalanced. From this perspective, theory might also be adjusted and modified to be more applicable in a peripheral society like Iran. with regard to the appropriateness of using second demographic transition for peripheral societies (like Iran), this strategic question may be raised: can a society enter second demographic transition period before completing her first demographic transition? In this article, we have found a positive answer to this question. Since, if we control for the effect of age composition, timing of replacement fertility will coincide with the timing of zero population growth. In other words, because of the effect of age composition, the second demographic transition in Iran overlaps with the first demographic transition for several decades.