The development of mechanical systems made of composite materials is presented. A relatively large four-axial 8-rotor composite unmanned Aerial Vehicle (UAV) and composite wind blades of a wind power system will be used as examples to explain the methods adopted in the development process. Regarding the development of the UVA, the UVA is designed to complete a prescribed flight mission for achieving the goals of consuming relatively low power and attaining high reliability. For weight saving reason, a 3D truss made of carbon/epoxy strands is used to fabricate the structure of the UAV. The fabrication processes of the composite truss as well as the composite UAV structure will be described in detail. The progressive failure of the composite UAV is studied using a geometrically nonlinear finite element method together with appropriate failure criteria to identify the failure loads and locations for different failure modes. The failure information obtained in the progressive failure analysis is used to establish a fail-safe design strategy for the UAV. The impact analyses of the UAV subjected to collision and landing, respectively, are performed to study the possible damages that may be induced in the UAV. The results obtained in the impact analyses are used to determine the allowable fly speed and design the landing gears for the UAV. The impact energy absorption capability of the adopted landing gear system is determined through drop testing. Based on the one-dimensional momentum theory and blade element method, the dimensions and geometric characteristics of the rotor blades are designed to produce sufficient lifting force to lift up the UAV with total weight (self-weight and payload) of 100Kg. A systematic method is then presented to evaluate the power consumption of the UAV for accomplishing a specific flight mission. The test results of the power consumptions of different parts in the UAV are used to estimate the power consumption of the UAV for accomplishing the flight mission. Flight tests are performed to validate the design feasibility of the UAV. A similar introduction of the process for developing composite wind turbine blades with high reliability will also be given.