Today the concept of antimicrobial coating has received great attention because of its potential to enhance safety. Poly(ethylene terephthlate) is often used as a coating due to its excellent properties. It is a light-weight thermoplastic polymer with a good impact resistance and outstanding processability. One of the methods that can be used for the improvement of the antibacterial properties of PET is chitosan immobilization. Chitosan is a natural biocompatible polysaccharide. The interaction between positively charged chitosan molecules on the amino groups and negatively charged microbial cell membranes leads to the leakage of intracellular constituents. The chemical structure of PET with very few available polar groups on the surface makes immobilization of chitosan difficult; so surface modification of this polymer is often required. In this research, we used chitosan grafting to make antibacterial PET films and two methods of carboxylation and partial basic hydrolysis were employed to modify films and introduce carboxyl groups which are negatively charged. Then two different molecular weights of chitosan were immobilized on the films through the reactions between the amino groups of chitosan and the carboxyl groups of the films. The presence of carboxyl groups on the treated films and the amino groups of the chitosan was identified using ATR-FTIR. The production of carboxyl groups on the hydrolyzed, carboxylated and native PET surface was determined titrimetrically by a TBO method and a calibration curve. Water contact angle studies were conducted to characterize the hydrophobicity of surfaces. Tensile and Water vapor transmission tests were employed to determine the mechanical behaviour changes of modified films and unmodified ones. Then antibacterial activity against gram-negative and gram-positive bacteria of all the prepared films was tested. FTIR and TBO tests showed that carboxyl groups were produced on the surfaces of both modified PET films. By comparing the amounts of carboxyl groups, it was found that with hydrolysis and carboxylation, the number of carboxyl groups became respectively, four and three times larger than untreated polyester; so with hydrolysis, more amounts of carboxyl groups were created on the surface. Water vapor permeability test showed that permeability values were increased in the hydrolyzed surface. The results of tensile test confirmed that loss of strength in the hydrolyzed PET was lower than the carboxylated one. With water contact angle tests, it was shown that carboxylation and hydrolysis significantly improved the hydrophilicity. Water contact angle for the chitosan grafted films were lower than the films before grafting. The results of FTIR tests showed that chitosan was effectively immobilized on the films. Antibacterial tests confirmed that the reduction percentage in bacterial count number after 24h for Staphylococcus aureus was up to 92.31% and for Escherichia coli , up to 88.35%. Films with low molecular weight chitosan showed more antibacterial activity against Escherichia coli and films with high molecular weight chitosan showed more activity against Staphylococcus aureus . Also it was shown that for both kinds of bacteria, hydrolyzed modification was more effective because pretreated films with this method had a larger amount of carboxyl groups. Total results confirmed that with hydrolysis, more chitosan grafting, antibacterial activity and also better mechanical strength could be obtained. Keywords antimicrobial coating, chitosan, pretreatment of PET, carboxylation, antibacterial activity