This paper presents an effective method to achieve altitude and attitude (pitch, roll and yaw) control of a helicopter in hovering and low-speed forward flight conditions with an approximate mathematical model. To facilitate control design, a simplified multi-inputmulti- output (MIMO) affine nonlinear model that describes the angular rate responses of a helicopter is derived. The controller consists of two separated parts: altitude loop and attitude loop. A PID controller is used in the altitude loop and a dynamic inversion controller is used in the attitude loop. To compensate the dynamic inversion error caused by modeling uncertainties and disturbances, an adaptive compensating algorithm is employed. Simulation results of a YAMAHA R-50 unmanned helicopter simulation model show that the algorithm is stable and robust with good tracking performance and decoupling capability.