Force estimation is one of the most important techniques in the robot control application. In this research, acceleration sensor based load-side disturbance observer (ALDOB) is proposed and developed by integrating an acceleration sensor on the load side and the optical encoder on the motor side in order to estimate external force. The enhancement of the force estimation was found to increase. Good estimation of force and robust force control could be obtained using ALDOB observer. The simulation results are provided to illustrate the performance of the proposed algorithms. A flexible robot model was established to simulate the robot dynamic in force control, and the results agree with the analysis.