Abstract: In this paper, a new robust adaptive control law, referred asθ-δ modification, is developed. Unlike other modification methods, the θ-δ modification does not degrade the performance of the control system; i.e., the new adaptive law guarantees that the tracking error and all other signals in the adaptive loop are asymptotically stable in the ideal case for which the system is perturbation-free. When the system is subject to perturbations, the adaptive law is robust, with a small drift in error between the plant and model outputs regardless of whether the constant γ is large or small. Therefore, the constant γ can be chosen arbitrarily.