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Aerodynamic forces, power consumptions and efficiencies of flexible and rigid tandem wings undergoing combined plunging/pitching motion were measured in a hovering flight and two forward flights with Strouhal numbers of 0.6 and 0.3. Three flexible dragonfly-like tandem wing models termed Wing I, Wing II, and Wing III which are progressively less flexible, as well as a pair of rigid wings as the reference were operated at three phase differences of 0, 90 and 180. The results showed that both the flexibility and phase difference have significant effects on the aerodynamic performances. In both hovering and forward flights at a higher oscillation frequency of 1 Hz (St= 0.6), the Wing III model outperformed the other wing models with larger total horizontal force coefficient and efficiency. In forward flight at the lower frequency of 0.5 Hz (St= 0.3), Wing III, rigid wings and Wing II models performed best at 0, 90 and 180 …