Adsorption and deposition of proteins in/on membranes were two main mechanisms of protein fouling in microfiltration. To distinguish the two mechanisms, adsorption and deposition were studied under both static and filtration conditions. The results demonstrated that under static conditions, adsorption had an equilibrium and the equilibrium amount of adsorption was almost constant even in solutions with different concentrations. The maximum adsorption occurred at isoelectric point pH value, and the amount of protein absorbed was similar to that of monolayer type adsorption. In actual microfiltration, adsorption and deposition of proteins occur simultaneously and the adsorption has an equilibrium with adsorption values similar to that of static adsorption. The amount of protein that deposited onto the membranes was simply proportional to filtration volume of the solution. The result can be described by a mathematical model.