This paper presents a formulation of robot manipulator inverse dynamics based upon the Newton-Euler equations and the zero reference position method. The angular velocities and accelerations, linear accelerations of the mass centers, and inertia forces and moments of each link of the manipulator are computed recursively starting from the base to the end-effector. But, the joint reaction forces and moments, and joint actuator forces or torques are computed recursively from the end-effector to the base. The formulation is at first carried out in the current position and then reformulated by directly utilizing the zero reference position data. The former formulation is intuitively simple; while the latter is computationally more efficient. A parametric study is also conducted to compare the relative efficiency of this formulation with the previously available formulation.