A new method has been developed for determining the average porosity of filter cake overlying the membrane in deadend ultrafiltration from only the flux decline behavior observed in two ultrafiltration tests without using any specially designed filtration cells. The validity of the method was examined for dead-end ultrafiltration of bovine serum albumin solutions. The method was based on a comparison of the ultrafiltrate flux data obtained from a concentrated solution with that obtained from a dilute solution. The average specific cake resistance was determined over a wide range of pressures from a single step-up pressure dead-end ultrafiltration test of dilute protein solution. The pressure dependence of the average cake porosity was evaluated by comparing these data with the infinite specific cake resistance data determined from a variable pressure ultrafiltration test of the concentrated solution. The porosity vs. pressure data were in reasonable agreement with those obtained from a series of constant pressure dead-end ultrafiltration tests performed at several pressures using a specially designed filtration cell capable of a sudden reduction in the effective filtration area. Furthermore, the flux decline behavior in constant pressure dead-end ultrafiltration experiments carried out at different pressures and solution concentrations was well evaluated based on the compressible cake filtration model, using the porosity data together with the specific cake resistance data. The simplified method presented here for easily estimating the porosity vs. pressure data from the flux decline behavior in dead-end ultrafiltration has important implications for analyzing the properties of filter cake formed in dead-end ultrafiltration.