A caseinomacropeptide (CMP) concentrate was produced from sweet whey by ultrafiltration (UF) and diafiltration (DF) using polyethersulfone (PES) membrane. Effects of the pH of whey feed and molecular weight cut-off of membrane (9 and 25 kDa) on permeate flux, recovery and purity of CMP were investigated. Before the UF, a pre-heat treatment at 90°C for 1 h and then acifidication to pH of 5 were applied to precipitate the major proteins in sweet whey. The pH value of UF feed was re-adjusted to different pH values (3, 4, 5, 7 and 9) to concentrate CMP molecule in the retentate and separate other whey proteins through permeate. Feed pH of 9 and 7 provided an adequate flux with 9 kDa- and 25 kDa-membrane, respectively. A volumetric concentration factor of 4 was reached with both membranes by UF and subsequent DF, but the process time was shorter with 25 kDa-membrane because of the higher permeate flux. One DF cycle was possible with 25 kDa-membrane as there was a substantial loss of CMP compared to four DF cycles with 9 kDa-membrane. About 58% of CMP in whey was recovered using 9 kDa-membrane while 33% of it was recovered with 25 kDa-membrane by the whole process. α-Lactalbumin, β-lactoglobulin, tyrosine and phenylalanine contents in the final concentrate, which are related to the purity of CMP were found similar in both cases. Sweet whey pre-treatment was the key point for purity of CMP concentrate before UF/DF. Both PES membranes can be used for the production of a CMP concentrate depending on desired recovery, composition and process time.