Steam generator water level control system is the most important components of the nuclear power plant. The operating steam generator water level controller is increasingly recognized as a capital asset that should be maintained and monitored routinely. However, this issue as the control loop performance assessment problem is still open. Considering this issue, a major contribution of this paper is to take the performance assessment technology into this industrial area, steam generator water level control system in nuclear power plants, where not much work has been done before. In terms of a class of steam generator water level control systems, two PI controller systems, performance assessment methods for the plant with stable and unstable zeros are developed at specific power level. An numerical example shows the effectiveness of the proposed method.