Making balance between the generation and demand is the operating principle of the load frequency control (LFC). Several studies in LFC have led to the trend of applying the adaptive approach to the implementation on automatic generation control (AGC). However, the adaptive controller with self-tuning technique requires online acquisition of system parameters that was not clearly stated in the related literatures. This paper proposes the online Recursive Least Square (RLS) estimation on system's parameters for an isolated power system. The estimated parameters not only could be used as a reference model for the design of the nominal gains in gain scheduling technique, but also could be applied to the online tuning of the controller gains under the time-varying condition. Simulation results show that the RLS based estimate with an adaptive controller could enhance the performance of the LFC.