In this paper, the algorithm for the formation flight is designed. In these days, UAVs are actively utilized in battlefield and civil area. In this point, many researches on multiple UAVs operation has been studied for various missions. In fact, formation flight gives various benefits such as high efficiency in aerodynamics and low exposure rate by opponent's radar. In this paper, the formation flight algorithm is designed based on the leader-follower method so that it is expandable to the change of number of operating UAVs. Virtual Pursuit Point concept is adapted to design guidance law and it needs only little computing power for real-time operation. Finally, the proposed formation flight algorithm is validated by MATLAB /Simulink environment simulation. Designed algorithm shows not only precise formation tracking performance but also expandability to the various number of operation UAVs.