The design of human assistive systems requires actuators that are capable of producing compliant motions. One key technological development for this compliant actuation is series elastic actuation, which is a challenging design problem, as the load side dynamics of the actuator changes during operation. In this paper, a novel series elastic actuator (SEA) structure using composite beams as series elastic elements and its torque control strategy are presented. We proposed a two-stage controller, a novel adaptive state feedback controller for transient dynamics and a PI controller for eliminating steady-state error. The effect of disturbance torque is studied, together with load dynamics, which is estimated online by a recursive least square method. The performance of the actuator was evaluated in case studies in which the actuator was controlled with various knee torque profiles. The experimental results showed that desired dynamic behaviour of the designed SEA can be obtained with the proposed control algorithm despite the unknown load dynamics.