In this study, the bacteriocin nisin at varius concentrations (50, 100 and 200 IU/ml) was studied as a bio-preservative to extend the shelf life of pasteurized milk. A minimum of one week extension in shelf life was obtained for pasteurized milk at refrigerator temperature storage. Higher concentration of nisin (200 IU/ml) showed no different effect than the lower one. The effect of nisin added on microbiological quality (standard plate count (SPC), psychrotrphs, proteolytic and thermophilic counts) of pasteurized milk studied and showed significant decrease in all of them than control samples (free from nisin). Also, nisin reduced the development of acidity so extend the shelf life of examined samples. The antibacterial effect of nisin studied on Staphylococcus aureus (Staph. aureus) inoculated in pasteurized milk and the result clarified complete inhibition of Staph. aureus at 6^th day. Palatability of addition of different concentrations of nisin was also discussed. Nisin had the ability to control the spoilage bacteria so could be control the acidity of products and have antibacterial effect against pathogenic bacteria so act as a principal, safe and natural preservative for pasteurized milk.