This work has advanced the understanding of flapping flight of flexible wings designed to be used in micro air vehicles. A complete new experimental setup that includes a wing actuation mechanism, a customized digital image correlation system, a control system, a load sensor and a vacuum chamber is realized for this study. The technique of digital image correlation has also been developed so that complicated wing kinematics and deformation can be measured. The flapping wing effectiveness and efficiency have been evaluated in different conditions. The results indicate that passive wing deformation can be utilized to enhance aerodynamic performance, under certain inertial loading mainly dictated by flapping frequency, amplitude, wing compliance and mass distribution. The wing deformation reflects the aeroelastic effect produced by the coupled aerodynamic loading as well as the inertial loading. Critical parameters extracted from the deformation data are used to characterize the structural properties of the wings and correlate with the aerodynamic performance. The correlation shows that for one-degree-of-freedom kinematics, wing deformation can be directly used to predict time averaged thrust. The intrinsic relationship between kinematics and inertial loading enables the design and optimization of wing structure based on the correlation results.