This thesis presents Load Frequency Control (LFC) and Automatic Generation Control (AGC) and its role and importance in interconnected power system. Supplementary frequency control, which is also known as load-frequency control (LFC), is a major function of AGC systems and one of the most important ancillary services, which are used by transmission system operator (TSO). We have developed simplified mathematical model of control area on Matlab/Simulink platform and with computer simulation we have investigated dynamic response and influence of different parameters and production structure of control area on the operation of the power system. With one year analysis we have investigated load-frequency control performance within a control area according to new Load Frequency Control and Reserves Network Code which is now merged in System Operation Guideline of ENTSO-E. We have analyzed influence of different parameters and production structure of control area on load-frequency control performance, activated reserve power and generation rate constraint (GRC). We have proposed recommendations, guidelines and limitation of the presented model for the use in practice and possibilities of improvements and upgrades of the model.