This paper considers the decentralized turbine-governor control design for the damping of low-frequency, inter-area oscillations in electric power systems. A time-varying model that explains oscillations caused by parametric resonance is employed to design a controller. A robust turbine-governor controller design is presented on the basis of the model. Furthermore, a basic idea for incorporating constraints on the amplitude of the turbine output is introduced. By using a two-machine-quasi-infinite-bus system, we demonstrate that the proposing control effectively damps low-frequency oscillation.