The current study aimed to extend the shelf life of pasteurized milk through addition of chitosan nanoparticles (CNP), which were obtained from chitosan by ionotropic gelation and sonication methods. Physicochemical characteristics of CNP were characterized by fourier transform-infrared spectroscopy (FTIR), zetasizer and transmission electron microscopy (TEM). FTIR spectra analysis of CNP shows a distinctive band at 1150 cm^-1 due to stretching vibration of phosphate groups. Size of the freshly prepared CNP ranged from 90 to100 nm but after two weeks of storage at room temperature the size increased to 120 -130 nm. By using TEM, freshly prepared nanoparticles appeared as spherical in shape with a little fragmentation of polymer chain and swelling but still translucent after 2 weeks of storage. Synthesized CNP at a concentration of 0.3% w/v has the ability to inhibit growth of vegetative cells of B.cereus in pasteurized milk stored at 5°C and extend the shelf life of pasteurized milk from two weeks to 30 days. The addition of 0.3% w/v CNP had no effects on the physicochemical properties, such as rancidity, pH and color, of pasteurized milk.