In this study, the influence of media-packing ratio on the performances of anaerobic hybrid reactors (AHRs) at low, medium and high organic loading rates was evaluated by conducting COD profile, granulation and tracer studies. Four laboratory up flow anaerobic hybrid reactors, each with a total unpacked volume of 7.85 I, with varying packing depths, were operated at organic loading rates from 1 to 24 g GOD/1 d. The media-packing ratios were 75%, 60%, 40% and 20% of the total reactor height in the AHRs. Three types of soluble COD profiles along the reactor height were observed when the organic loading rate was gradually increased. When operated at 1 and 2 g COD/1 d the COD profiles along the reactor height from bottom to top showed a plug-flow regime. From 4 to 12 g COD/1 d the COD profiles were distorted in the reactors with 20%, 40% and 60% packing, while at 16 g COD/1 d and above the COD protile indicated homogeneity in each reactor, suggesting a perfectly-mixed regime. The distorted COD profiles were considered to be caused by the non-ideal flow pattern prevalent in the reactors. The dead-space volume and the bypass flowrate due to short-circuiting were determined using the Cholette and Cloutier model. A ＇distortion index＇ (DI), which was calculated from the ratio of the average COD value of the sludge bed over the average COD value of the reactor, was used to describe distortion of the COD profile. The distortion index correlated well with the short-circuiting fraction.