One of the most important factors affecting the performance of a pervaporation process is the type and nature of the membrane used in this process. One of the characteristics of different polymeric materials used in manufacturing these membranes which can affect the membrane properties is the molecular weight of the polymer. In the present study the effect of molecular weight of poly (vinyl alcohol) on the membrane performance in pervaporation dehydration of IPA/Water mixture was studied. For this purpose, poly (vinyl alcohol) with three different molecular weights (30000, 60000, and 145000) was used for membrane synthesis where polymer crosslinking was carried by glutaraldehyde (at three different amounts) in the presence of hydrochloric acid as catalyst. Membranes structure and other properties were characterized by different methods. To determine the membranes performance, separation tests were carried out for dehydration of 80 wt% aqueous solution of isopropanol at three different temperatures (40, 50, and 65 degree Celsius). Membrane performance was investigated by measuring permeation flux and alcohol concentration in the permeated product. According to the obtained results the membrane made with poly (vinyl alcohol) with 60000 gr/mol molecular weight, using the lowest amount of glutaraldehyde, had the highest flux equal to 0.1237 kg/m 2 .h. On the other hand the membrane made of poly (vinyl alcohol) with 145000 gr/mol molecular weight, using the highest amount of glutaraldehyde, showed highest selectivity of 4978. Both of these values were obtained at 65 o C. Investigation the degree of swelling of membranes at constant amount of crosslinking agent showed that by increasing poly (vinyl alcohol) molecular weight, which results in chain length increasing, leads to increase in crosslinking between chains. Investigating the effect of process temperature showed that both permeability and selectivity of membrane with 145000 gr/mol molecular weight increased by increasing the temperature whereas in the case other membranes made from polymers with 30000 and 60000 molecular weight the variation of permeability and selectivity with temperature were in opposite directions. Keywords: Pervaporation, Poly (vinyl alcohol), Molecular Weight, IPA/Water Mixture, Permeation, Selectivity.