The population decline in recent years that have taken place as a result of the lifestyle changes, is influenced by modernism and modernist principles. The importance of studying this subject is realized when we know that if this trend continues, we will face problems caused by the aging population. Hence, the present article seeks to address this question: What is the role of modernity in changing lifestyles and population decline in Iran?Adopting an analytical- descriptive method and using historical, sociological sources, this article studies the subject in question. The conclusion drawn out is that although different economic, cultural and social factors are said to be involved in the population decline in Iran, an important factor named modernity has been neglected to a great deal. Enumerating aspects of population decline such as changes in ideological vision, promoting consumerism, rising marriage age and divorce, underestimating parental rolewhich have occurred as a result of modernistide as spreading in society is another achievement of this article.