The development of flapping wing micro air vehicles (MAVs) have yielded remarkable progress over the last decades. Achieving high component stiffness is often in conflict with low weight requirement, which is highly desirable for longer flight time and higher payload. Moreover, the vibration originated predominantly from the wings, the gears and the frame excitations, compromises the flapping wing MAV's stability and the fatigue life. In order to improve the vehicle's efficiency and performance, optimization of these various parameters is necessary. In this work, we present the structural optimization of the flapping wing micro air vehicles. Specifically, we focus on the gearbox optimization using Simulia Tosca Structure in Abaqus, which is a robust tool for designing lightweight, rigid and durable components. The various numerical experiments have been conducted towards optimizing the components' topology. The results provide understanding of the optimal design topology for a spur gear and the structural components used in MAVs.