Abstract: Conceptual approach for the design of gun launched UAV is given, taking the constraint of size of the stowed shell into account. Two kinds of objective function are derived. Of which the one is to seek the maximum payload under the endurance requirement constraint; and the other is to optimize the endurance with the required payload weight as a constraint. Using this method, a series of UAV equipped in 155mm caliber is developed, including 4 kinds of configurations. By optimizing the payload weight, characteristics of these configurations are analyzed and compared. Both the limiting payload and the maximum endurance time with the caliber are calculated.