A robotic force controller is usually designed in terms of an exact system model, such as the modified computed torque controller developed by McClamroch and Wang. If the mathematical model of the robot system is exact, the modified computed torque can simultaneously control the robot motion and the contact force in an accurate way. However, there may exist uncertainties in the model, such as the flexibility of joints and links, the joint friction, and an inexact surface model. It will be shown that the modified computed torque controller may result in an unstable closed-loop system for a system with uncertainties. This difficulty can be overcome by introducing a robust controller. Two robust force controllers are proposed. One is based on the non-singular model system so that the force is indirectly controlled. The other is based on the transformed system model so that both the robot motion and the force can be simultaneously controlled. The controllers are robust in that they are insensitive to variations in the plant parameters and to external disturbances of contact force.