The paper proposes an adaptive speed/position tracking control of an induction motor subject to unknown load torque. The controller is developed based on a dynamic model obtained from the d-q-axis model (w.r.t. the stationary reference frame) of the motor under a special nonlinear coordinate transform so that either the speed or position control objective can be fulfilled. The underlying design concept is to endow the closed-loop system with a so-called maximal power transfer property while under lack of knowledge of some key system parameters, such as the rotor resistance, motor inertia and motor damping coefficient. To be rigorous, the proposed control scheme comes with a thorough proof derived based on Lyapunov stability theory. Numerical simulations and experimental results are also given to validate the effectiveness of the presented control scheme.