The most widespread method applied for standardization of milk protein in cheese production is ultrafiltration. In the current study, pasteurized, unhomogenized whole milk was ultrafiltered with APV ultrafiltration system, 51 spiral wound, polysulfone membrane with 20000 Da molecular weight cut off (DDS, Nakskov, Denmark) at 50oC, such that the pressure difference between the two sides of membranes was not more than 3.6 bar. permeate that was collected, contained (w/w%) 0.01 % proteins. The protein was isolated from crystalline TCA to the concentration of 12.5% (w/v), and then characterized by discontinuous SDS-PAGE (10-20% acryleamide). The products of proteolysis contained a-lactalbumin (49.95±2.25 %), b- lactoglubolin (22.30±2.3%) and casein proteolysis products (30±4%) and (96.68±1.725), (99.18±0.46) and (98.39±1.005) % were rejected by ultrafiltration membrane, respectively. Casein micelles were rejected completely (100%), therefore, they couldn’t penetrate to permeate from retentate. Incomplete rejection of casein and penetration of casein into permeate indicates the leakage of retentate in to permeate.