Insurance is one of the most important domain, which can be studied, and developed through probability theory and statistical tools. Mostly if we speak about relationship between insurance and statistics, firstly we think about life-insurance, due to a lot of indicators that have to be analyzed, such as demographic phenomena, mortality etc. The specify of life-insurance is that insurance company and it is client deal with long term contracts, that suppose for each of them some kind of objectives. Client has a object to be fully protected, in time that insurance company tries to create and keep some financial reserves. In such away, insurance company, has to calculate specific premiums, in order to cover current contracts liabilities and in the same time to deposit a part of premium to reserve funds. In actuarial analysis are two methods to calculate mathematical reserve of lifeinsurance: retrospective and prospective. From the retrospective point of view the terminal reserve for a given policy is defined as the difference between accumulated value of past net premiums and accumulated value of past insurance benefits. From the prospective point of view the terminal reserve for a given policy is defined as the difference between the value of the future benefits of the policy and the value of the future net premiums. It is important to show that the retrospective and prospective methods of obtaining a given reserve always produce identical results consider the fundamental equation used in determining the net premium.