Abstract: An adaptive learning control scheme is designed for first-order nonlinearly parameterized systems with unknown periodically time-varying delays. It is assumed that the common periodicity of unknown time-varying parameter, time-varying delay, and reference signal are known. By reconstructing the system equation, all unknown time-varying terms including the time-varying delay are combined into a periodically time-varying vector which is estimated by a periodic adaptation mechanism. By constructing a Lyapunov-Krasovskii-like composite energy function, we prove the boundedness of all signals and the convergence of tracking error. The results are extended to a class of high-order nonlinear systems with mixed parameters. Two simulation examples are provided to illustrate the effectiveness of the control algorithms proposed in this paper.