Stability is indispensable to haptic interfaces for the simulation of a large variety of virtual environments. On a multi-degree of freedom (multi-DOF) haptic device, the passivity condition must be satisfied in both end-effector and joint space to achieve stable interaction. In this study, a conservative passivity condition is utilized for the stability such that guaranteeing the passivity at all joints is a sufficient condition for the passivity and then stability of the whole haptic system. An optimal posture control algorithm is developed to satisfy this passivity condition and maximize the stability performance of a redundant haptic device. The algorithm optimally adjusts the device postures, which are estimated by a Golden Section Search algorithm. The proposed control algorithm was experimentally implemented on a virtual sphere by using a 7-DOF redundant haptic device. Z-width stability metric was used to evaluate the performance of the proposed algorithm. The results show that the optimal posture control approach significantly improves the stability of the redundant haptic devices.