This study explores the effect of semi-active friction damping on vibration isolation systems by means of numerical simulation. Since the Lugre friction model can describe many phenomena observed in laboratories, it is chosen to provide accurate friction force model for the analysis and simulations. The drawback of the passive friction damping system is that it decreases the resonance response at the cost of worsening the performance for high frequencies. A semi-active control method (skyhook method) is used to tune the normal force so that the energy dissipated by the friction force is maximized. Such semi-active friction damping system is simple but efficient, and it can achieve result almost as good as that of active systems. Results from numerical simulations show that compared with passive system, semi-active control method decreases the response significantly for both low and high frequencies, with harmonic and random excitations.