In this study, Alumina-PDMS nanofiltration membranes are fabricated and the effect of PDMS OH-treatment and molecular weight is investigated on salt rejection. Pressing is used to prepare the alumina substrate and PDMS layers with various M(w)s are coated on the substrate through dip-coating. Also, characterization is performed using FE-SEM, X-ray diffraction, ATR-FTIR and MWCO techniques. Results show that higher M-w PDMS shows enhanced efficiency due to better coverage on the substrate. Membrane rejection with the 5000 cSt PDMS plus 750 cSt OH-terminated PDMS was obtained as 75%, 67% and 35% for Na2SO4, MgSO4 and NaCl, respectively. The average pore size is also calculated as- 1.75 nm using MWCO technique. Based on the results, PDMS can be proposed as a proper option for development in desalination processes.