In this paper, we study the influence of a haptic interface's effective mass and viscous damping on the hardness perception of simulated virtual objects in six-degree-of-freedom (6-DOF) haptic rendering. It is found that induced forces from these physical parameters affect the perceived hardness nonuniformly as users tap virtual objects with different parts of the tool. Fundamentally, the distinct hardness perception is caused by the discrepancy between the user's holding point and the active end of the device. A stick-on-ball application was used for comparisons of three proposed compensation methods. A total of six stiffness intensities in the range of 0.1-0.6 N/m at a step of 0.1 were randomly presented to a total of ten test subjects. Experimental results show both the need for compensation and the effectiveness of the proposed methods.