Tendon sheath mechanism (TSM) is an effective power transmission system to access remote locations through tortuous channels, and it is widely used in endoscopic surgical robots. However, owing to the deformation of TSM during power transmission, the input-output hysteresis is expressed in the form of backlash-like hysteresis. This paper is premised on the idea that sheath deformation can occur if the sheath is fixed only at certain points rather than being fully fixed at all points. Based on our hypothesis, a new TSM-pair backlash-like hysteresis model was derived that considers sheath stiffness. Experimental setups were designed to validate the new model, and its mechanical parameters were identified. The experimental results revealed that the sheath significantly deforms and that such deformation produces backlash-like hysteresis together with tendon deformation. This model can be used to provide accurate prediction and control of TSM.