Nowadays, plastics are extremely important in packaging industry and they are being used more than other products. However, these materials have many advantages,but one of their biggest disadvantage is their slow biodegradation characteristics and environmental pollution. Biodegradable polymer can be degradated in natural environments by enzymes and chemical changes in their structure. Poly Lactic Acid is the most commonly used biodegradable polymers and competes well with common plastics used in food packaging industry. oly-Lactic Acid can be used in various food packaging and with the addition of silver nano- particles within their structure can be converted to antimicrobial active packaging products. Nano-silver are particles with dimensions less than 100 nm. These particles develop the antimicrobial activity, antifungal and biochemical changes. The purposes of this research are to produce a Bio-nanocomposite containing silver nanoparticles and investigate the possibility of increasing the shelf-life of pasteurized milk by this composite and measurig the amount of silver ions migrate into milk.. In this study, Poly Lactic Acid and polyethylene pellets were mixed with a mixture containing 90% titanium dioxide plus 10% silver nano-particle by using extruder. Then, the films with 0.5,1 and 2 percent compound containing silver nanoparticle were produced with using the hot press machine. Then the antimicrobial activity of the films on both Gram positive bacteria ( Staphylococcus aureus ) and Gram negative bacteria ( Escherichia coli ) was tested. Next, the highest antimicrobial activity was used to reduce microbial load and increase the shelf-life of pasteurized milk and pasteurized milk contain E. coli bacteria. Also, the total number of microorganisms and coliforms were counted at 0, 3, 7 and 14 days of storage. According to the result, all three compound containing silver nanoparticles added to polymer could significantly reduce the number of both Gram positive and Gram negative bacteria. Also, the result showed nanoparticles had more antimicrobial effects on Gram negative compare to Gram positive bacteria. In the later part of this study, Poly Lactic Acid and polyethylene nanocomposites with 2% composition of silver nanoparticles which had the highest antimicrobial activity could significantly reduce pasteurized milk pathogens than the control sample pasteurized milk until fourteenth days of storage at refrigerator. Furthermore, the number of milk pathogen was less than the acceptable limit until seven days. The results of the antimicrobial effect of nanocomposites treatments on milk contained E. coli bacteria showed that they were able to significantly reduce the number of microorganisms compare to the control sample. Also, more antimicrobial effect of nanoparticles was seen on the reduction of E. coli bacteria than the other organisms in milk. The migration of silver ions into milk during storage was less than the allowable toxicity. Key words : Poly Lactic Acid, Silver nanoparticle, Pasteurized milk, Nanocomposite.