A novel strategy for preparing low-fouling ultrafiltration membranes with tunable molecular weight cut-offs (MWCOs) from microfiltration membranes as substrates was demonstrated. In this method, carboxybetaine polymers were grafted onto the surface and pore walls of the microfiltration membranes that had pore sizes in the tens of nanometers. One of the roles of the grafted carboxybetaine polymers is to give low-fouling property, and the other is to reduce and tune the pore sizes. The plasma graft polymerization method was employed to graft the carboxybetaine polymers, and the characterization using FT-IR and FE-SEM demonstrated that the grafting was successful. We then demonstrated the grafted membranes showed excellent low-fouling properties against not only bovine serum albumin but also dextrans using the adsorption tests and the permeation tests. Additionally, we showed that the MWCOs of the ultrafiltration membranes prepared in this method can be tuned in the range from 5 x 10(3) to 5 x 10(5) by the grafting amounts.