Abstract: Antimicrobial modified polystyrene (PSPHMG) was prepared via melt reaction between polyhexamethylene guanidine (PHMG) and epoxyterminated polystyrene in a Haake torque rheometer. FTIR spectrum confirmed that the oligoguanidines had been successfully connected to PS macromolecular chains via chemical bonds. Diffusion method and shaking flask method were used to measure the antimicrobial activities of synthetized PSPHMG against E. coli and S. aureus. Diffusion experiments showed that there was no dissolution of guanidinium oligomers from the extracted PSPHMG. However, the inhibition zones can be also found, indicating the abtimicrobial activities against E. coli and S. aureu. When the mass fraction of PSPHMG in the PS matrix was 0.2%, and contact time was 30 min with E. coli, the growth inhibition was 100%. When the mass fraction of PSPHMG reduced to 0.1%, contacted 30 min with S. aureu, the growth inhibition of 100% was also obtained.