In a two-phase flow system, if the relative permeability curves areplotted in terms of the saturation of each phase, it is observed that thesecurves of each phase differ during the drainage and imbibition processes, whichis called hysteresis and some parameters such as pore size distribution andwettability of the rock have an effect on it. Knowing the relative permeability of each phase during bothprocesses of drainage and imbibition is very important in EOR studies. So far,many studies have been performed on sandstone reservoirs and several modelshave been proposed to predict the relative permeability hysteresis for them.These models include the Carlson and Killough models, which have been widelyused and showed a good match with laboratory results. But over the years, verylittle research has been done on carbonate rocks, and considering that the mostof the reservoirs located in Iran are composed of carbonates, the study ofhysteresis models for carbonate rocks is vital. One of the results obtained in this studywas that the hysteresis in relative permeability bounding curves for the waterphase in neutral and oil-wetting samples was higher than that for the oilphase. Also, by changing the wettability from neutral to oil-wetting, thedegree of hysteresis of the bounding curves for the oil phase does not changemuch. In this study, we evaluated theexisting models for predicting the relative permeability scanningcurves incarbonate rocks and compared them with laboratory results and found that the predictionerror of Killough model was lower than Carlson model and increased afterchanging the wettability from neutral to oil-wetting. In the following, wepresented the modified Killough model, the amount of prediction error in thismodel was reduced relative to Killough model and the match with laboratoryresults was improved. Land trapping model presented a better prediction thanSpiteri model in both neutral and oil-wetting conditions. Land model predictionerror was lower than Spiteri model and more consistent with laboratory results. Keywords: Hysteresis, Relativepermeability, Carbonate rocks, Saturation functions, History matching