Galactosyl-oligosaccharide (GOS) is a prebiotic carbohydrate, produced from lactose hydrolysis, that serves as a value-added functional food element for humans. Now GOS can be produced through enzymatic reaction with lactose either in batch mode or with immobilized enzyme on membrane. In the second case, the main concern is the fouling of the membrane that could reduce both the GOS yield and purity. The present study is thus an attempt to project the superiority of a membrane reactor called the rotating disk membrane bioreactor (RDMBR) over batch mode to obtain purified GOS with high yield. It was found that GOS yield and purity were 32.4% and 77% respectively in batch mode followed by diafiltration-assisted nanofiltration. However, in the immobilized state they were 67.4% and 80.2% at 105 rad s(-1) membrane speed. Retention of monosaccharides that inhibit enzyme in the reaction volume of batch mode reduced the yield of GOS. On the contrary, simultaneous production and purification of GOS in RDMBR led to a high yield of GOS.