By the end of the Cold War and bipolar division of the world the term security is redefined. Modern concept of the security is no longer defined primarily in the military terms of force and power between states in international relations. Appearance of the new sources of threats has brought issues in discerning importance and severity which specific aspects can represent to the fundamental national interests and values. By the end of 20th century significance of demographic development and it’s fundamental elements (natural and mechanical movement of population and population structure) becomes more clear in stable functioning national and international systems and their further development. Therefore, unfavorable demographic tendencies such as aging population, natural depopulation and mechanical depopulation become more important in scientific and professional analysis of security threats. Many Croatian demographers emphasize that demographic development of Republic of Croatia is characterized by negative demographic processes and trends which seriously threaten its national system. Consequently, the aim of this paper is to establish security aspects of demographic development of Republic of Croatia and their influence on national system.