In this study, two definitions of critical flux were introduced based on the cross-flow microfiltration (CFMF) experiments conducted under an operational mode of constant permeate flux. The critical flux based on mass balance was calculated from the rate of particle deposition at a certain permeate flux. Below the critical flux, there will be no particle deposition on the membranes. The other was defined on the basis of the increase in transmembrane pressure (TMP). The critical nux based on the TMP increase is the flux below which membrane fouling does not occur. The critical nux based on both definitions was evaluated from experiments with different particle size, membrane pore size? influent concentration and ionic strength of the influent suspension.