The responses of structures under seismic conditions present very high nonlinearity due to both the parameter variations of the structures and the nonlinearities of the forces induced by ground motions. On the other hand, an adaptive controller (stabilizer) can adjust itself continuously to the changes in the system. Therefore, this paper examines the applicability of the adaptive controller to the nonlinear seismic structures so as to enable us to design economic and attractive structures which can successfully withstand the earthquake loading. The approach to the desired active control employs model reference adaptive control (MRAC) technique. Initial parameters of the system, which are mass matrix, damping matrix and stiffness matrix, are identified using off-line algorithms. An adaptive controller is, then, introduced to change the system characteristics of the target structure so that the responses of the controlled structure, under seismic conditions, will follow the expected ideal responses which are the output of a reference model. The reference model is constructed according to the specifications on the performance requirements of structures under earthquake excitation. It is shown that the MRAC technique can be applied to solve the active control problem of nonlinear seismic responses of structures in an effective and efficient fashion. The controller proposed demonstrates rational self-tuning capability to the random changes of earthquake excitation as well as to the time-varying property of nonlinear seismic structures under strong- motion earthquake. The theoretical results obtained are illustrated and verified with numerical examples.