This brief describes the mathematical model for shape memory alloy (SMA) actuator based on the physics of the process and presents a model-based sliding mode control (SMC) law. It is noted that only the contraction displacement can be obtained by the encoder. Whereas the unmeasured states can be estimated using an unscented Kalman filter (UKF) algorithm based on the nonlinear mechanism model. The comparisons between SMC incorporated with an UKF and SMC show that the tracking results of the proposed method is better than the previous one. Meanwhile, the load disturbance rejection result verifies the proposed controller has the ability to compensate the disturbance and ensure the robust stability property.