A pair of dynamically scaled robotic dragonfly model wings was developed to investigate the aerodynamic effect of wing-wing interaction in dragonfly flight. Instantaneous aerodynamic forces were measured while forewing-hindwing phase difference (gamma) was systematically varied. Experimental results showed that, (i) for hovering flight, gamma=0deg enhanced the lift force on both forewing and hindwing; gamma=180deg reduced the total lift force, but was beneficial for vibration suppression and body posture stabilization. In nature, 0deg is employed by dragonflies in acceleration mode while 180deg is usually in hovering mode. (ii) For forward flight, wing-wing interaction enhances forewing lift while reduced hindwing lift at all phase differences. Furthermore, the total lift was slightly reduced for gamma= 0deg to 90deg and significantly reduced by 18% when gamma=270deg. The results consist well with the fact that, dragonflies usually employ 50deg to 100deg for forward flight, but seldom employ 270deg. PIV results are shown for wing-wing interaction analysis.