To study the effect of transmission stiffness on the stability of a haptic display in virtual reality, a one-DOF(degree of freedom) haptic display system is modeled as a two-port electric network, and its absolute stable condition is provided according to two-port stable theory. Stability of a haptic display system can be evaluated by the achievable spring rigidity when displaying a parallel spring and damping virtual environment. The suggested absolute stable condition meets well with what is already generally accepted when the transmission stiffness trends to be infinite. Lastly the PHANToMDesktop is equivalent to be one-DOF on the manipulating space, and method for analyzing the stability and dynamic isotropy of analogous multi-DOF device is provided. Experimental results based on PHANToMDesktop show that the transmission stiffness has large effect on system stability.