Active disturbance rejection control (ADRC) has gained significant attraction due to its simple tuning method and robustness against process parameter variations, whether the plant is linear or non-linear, time-varying or time-invariant. In load frequency control, the use of ADRC is proposed to regulate the area control error, frequency error and tie-line error in the presence of internal dynamics and external disturbance. This paper presents a study on the effects of state observer bandwidth, controller bandwidth and sampling period on the performance of the ADRC based load frequency control of single- and multi- area power systems. The study gives good insight in choosing controller parameters for ADRC based LFC.