This article described a control law for stabilizing the vertical motion of a flapping-wing MAV and developed a system architecture that is potentially beneficial in realizing the autonomous flight of flapping-wing MAVs fewer than 10 g. The article began with a brief introduction to the Golden Snitch, including its development history and the development of the overall system. The vertical dynamics were given for altitude control. The use of wind tunnel tests to obtain aerodynamical parameters was described. Due to the limited payload-carrying capability, the control architecture was modified so that automatic control of flight altitude of a flapping-wing MAV fewer than 10 g is possible using current technology. Taking the hardware constraint into account, it was shown that the modified P-control can stabilize the vertical motion and track altitude commands. Numerical simulations and flight tests were presented that demonstrate the function of the developed control law and the system architecture. We believe this flapping-wing MAV to be the first under 10 g able to automatically maintain its flight altitude.