The most important concern about an atmospheric ammonia storage tank is the release of the large amount of ammonia due to the loss of tank integrity. Recently, several ammonia storage tanks have been built in Iran which necessitates a deep study about the failure risk associated with them. In this paper, fault tree analysis as a quantitative risk assessment has been applied to specify and evaluate the related risks of ammonia storage tank with the capacity of 20,000 tons. Five major causes for the ammonia large release are specified as the top events. Based on these top events, all possible sub-events therefore are recognized and the fault tree analysis is performed. In-house software, MECHREL, is prepared to evaluate the risks related to the specified storage ammonia tank. Moreover, the sensitivity of each event is investigated in the following. These results can be used to identify the most critical paths quickly to have reasonable risk-based decisions.