Eli Ltd. and Tallinn University of Technology (TUT) Department of Mechatronics are currently performing studies in order to develop a mini-class universal purpose unmanned aircraft [1,2]. The paper focuses on strength calculations and weight vs. strength optimization of the fuselage of this developed UAV system. To develop a strong but lightweight UAV fuselage, advanced computer modeling and finite element structure analysis are used as virtual prototyping tools for the optimization of the fuselage at early design stage and through the production period to improve the design . Design optimization is applied to minimize the maximum stresses within the fuselage, subject to strain constraint in conjunction with both geometry and choice of appropriate fibre orientations and stacking sequence as design variables and also material parameters. The fuselage for the UAV plane was designed and manufactured using E-Glass/Epoxy and High modulus (HM) carbon/Epoxy composites. In this paper ANSYS software has been successfully applied to minimize the weight of the fuselage and increase the UAV fuselage strength. The results show how the fuselage design could be improved with the help of finite element method analysis and provide guidelines for the structure and material design for the composite based UAV SWAN fuselage.