Abstract: In this paper, the problem of multivariable linear optimal regulator with quardratic performance index is discussed in frequency domain by using the output feedback method.The frequency domain result of optimal control is obtained by combining feedforward decoupling control of multivariable system with the optimal control. Not only can this optimal control law make the loss of energy and the error of output jointly optimal, with a relatively strong disturbance resis tance, but also satisfy the specified performance index in time domain. The method is simple for use and convenient to engineering application. As a example, an optimal design is made in this paper for a vertical flight control system.