Free vibration analysis program for rotationally periodic structures is coded. Many kind of structures have the form of rotationally periodic structure such as cooling tower of the nuclear power plant, large chimney, oil tank, natural gas tank and tire. The rotationally periodic structures can be divided into several substructures with the same form called the sector. Using this property, one can obtain natural frequencies and the corresponding mode shapes of the entire structure with information of only one sector as shown in Ref.[1,2]. It is programmed, called as sector method in this paper, and is added to ADINA the commercial FEM package. One can efficiently calculate the dynamic characteristics of the rotationally periodic structure by using the modified program. The program can quickly give the accurate results with very small memory space of a computer. As an example to demonstrate the efficiency of the program, cooling tower of the nuclear power plant is considered.