The social security organization provide a wide range of services and each has its own content and complexity. Providing an appropriate model for calculating the cost of services, helps the managers in decision making and planning to improve existing service provision and to make better evaluation of operations to increase efficiency and effectiveness of various activities. In this research, we try to identify and categorize different types of provided services in the long term payments Unit of branch no. 22 in Tehran and to determine and categorize the cost centers in it, and design an activity-based costing system to determine the cost of services providable in this unit. In order to understand the activity centers, determine the cost motivators and confirmation of the proposed model, Delphi technique is used and Delphi technique data, analysis and results are used to design a costing model. The proposed system is designed in eight stages, first the departmental costs are divided into administrative and personnel costs. Then the activities which have a share in these costs are identified. Subsequently, by using Delphi's data collection procedures, specific cost motivators are identified and costs are linked to activities. Finally, by determining the appropriate motivators, allocation of costs to services (retirement pension, disability and death) are done.