Microfiltration (MF) is the separation process which casein micelles could be recovered from skim milk mainly by size exclusion. Our objective is to find out the influence of serum protein on rheological properties and Sol-gel transition of CM dispersions by rheometric measurements at 20 °C in order to better understand their filtration behavior. A parameter (R) is introduced in this work as the ratio of serum protein to casein micelle and a range of values for this parameter are considered to analyze the rheological behavior of mixed protein dispersions. It was concluded that: firstly, the viscosity of mixed dispersions increases by increasing the concentration. A change from Newtonian to shear thinning is evidenced at increasing concentrations. Secondly, by increasing the ratio (R) the viscosity will decrease, sol gel transition zone will be shifted to higher concentration and effective volume fraction will reduce.