The fascinating flight traits of butterflies, such as low wing loading, low flapping frequency, and the ability for long-distance flight, have intrigued scientists and engineers. To study these mechanisms, a 2.7 g butterfly-inspired flapping-wing micro air vehicle (FWMAV) was developed using a combination of a single motor, elastic bands, a driving shaft, shape optimization, and a butterfly-like wing planform. During tethered experiments, aerodynamic forces and wing motions were simultaneously measured to understand the aerodynamics of the butterfly-inspired FWMAV. The results show that the ranges of resultant flapping and lead-lag motion of the wing of the robot were within those of real butterflies during the first stroke. The lift varied with the flapping motion and was sufficient for flight but averaged to zero over one flapping cycle because the opposite direction of lift was generated during the upstroke. These findings indicate that the modulation of the resulting aerodynamic force direction due to the control of the stroke plane angle per stroke plays a crucial role in the flights of butterflies and butterfly-inspired FWMAVs.