The position and attitude controls of flapping wing flyers are challenging because of their inherent instabilities. Insects can cope with such difficulties by finely and quickly tuning their wing kinematics. In addition, it is known that insects change their posture through the joint between thorax and abdomen in response to visual stimuli. In this study, the effect of the body flexion on the flight dynamics of a hovering hawkmoth are investigated numerically by using an in-house computational fluid dynamics (CFD) and a flexible body dynamics (FBD) solvers. For an integrated understanding of the effects of the body flexion, the curved or flexible body models, which replicate the longitudinal active and passive body flexion respectively, are developed. Our computational results indicate that the slight change of the center of mass (CoM) caused by the active body flexion alters the total aerodynamic torque, which result in the large pitch-up or pitch down of the body within a few wingbeat cycles. It is also found that, even though the rigid body pitches up in free-flight with a measured wing kinematics, the mild flexibility in the body can maintain the body attitude without any control. These results point out the importance of the CoM position on the flight dynamics and control of a flapping flight and, furthermore, the possibility of the simple but effective flight-control system with the body flexion for a bio-inspired MAV.