Membrane fouling by protein mixtures during microfiltration has been investigated for binary mixtures of bovine serum albumin (BSA), casein, lysozyme, pepsin, and ovalbumin. Filtration experiments were carried out using 0.2 μm polycarbonate track-etched (PCTE) membrane in a stirred cell under constant transmembrane pressure (14 kPa) and concentration of hydrogen ion (pH=11) to study the effect of mixture composition on filtrate flux decline. Flux decline data were analyzed using a pore blockage-cake formation model developed recently. It was found that the model is in a good agreement with the experimental data. Fouling parameters such as the rate of pore blockage(α), the initial resistance of the protein deposit (Rpo) and the increasing rate of the protein layer resistance(β) were used to evaluate the rate of filtrate flow by membrane fouling in the binary mixture system. Generally, the trend of α is comparable with that of filtrate flux decline. It was also found that fast flux decreasing was observed over the binary mixture containing casein. The result is due to high value of the initial resistance of the protein deposit (Rpo) over casein.