This letter presents a novel approach to the load-frequency control (LFC) of interconnected power systems. Based on functional observers theory, quasi-decentralized functional observers (QDFOs) are designed to implement any given global PI state feedback controller. The designed functional observers are decoupled from each other and also of low-order; thus, they are cost effective and easy to implement. Although the proposed approach is applicable to $N-$ area power systems, an example of a two-area interconnected power system with reheat thermal turbines is considered for simplicity. © 1969-2012 IEEE.