The dynamic ultrafiltration process of protein solution is experimentally investigated by measuring electrical conductivity within the filter cake. Three pairs of platinum wire electrodes were employed to measure the formation process of the filter cake on the membrane surface. The filter cakes formed from protein solutes behave as highly compressible fakes and shifting of pH from 5 to 4 or 9 results in a decrease in cake compressibility, as well as specific filtration resistance. The experimental results for bovine serum albumin and soy protein solutions coincide favorably with the calculated results based on the compressible cake filtration theory.