A multivariable theory is applied to the design of the recirculation and feed water control systems of a boiling water reactor. The nuclear steam supply system simulation model is represented by a set of 14 first-order nonlinear ordinary differential equations. Linearization about a steady-state operating point leads to a linear model. To reduce the computational effort, a sixth-order model is obtained by an optimal order reduction method. A robust controller is designed so that the regulation and tracking capability holds in the presence of certain perturbations and disturbances to the plant. The controller parameters are obtained by the parameter optimization method. The results of simulation show that the control system performs well and has robust stability.