Increased miniaturization of surgical instruments is essential to successfully perform surgical procedures in restricted areas as in many applications of minimally-invasive surgery. Miniaturization permits increase in dexterity and decrease in access incisions which are required in many sur- gical procedures. Tendon-based transmissions provide several important advantages for the mechanical design of miniaturized surgical devices. Reflected mass and inertia are reduced since tendon-based transmissions allow to locate the motors far apart from the actuated joint. In spite of providing several important advantages, they introduce several non linear effects that must be considered and modeled to achieve suitable performance. In this paper tendon-based transmission system for surgical robotics is illustrated. And nonlinear friction, due to direct sliding of synthetic fiber cables over small fixed pulleys (pins) is discussed and models aiming at compensating these nonlinearities are presented.