Background: Carboxymethyl chitosan (CMCh) is a water-soluble chitosan derivative and it has been widely used in biomedical applications due to its unique properties. Modification of CMCh by graft copolymerization with different monomers is done in order to enhance its applications. Results: Graft copolymerization of styrene (S) onto CMCh using ammonium persulfate (APS) as initiator was successfully done with optimization of grafting parameters. The new materials were prepared in nano size. The antibacterial activities against Streptococcus pneumonia (RCMB 010010), Staphylococcus aureus (RCMB 010028) as Gram-positive and Escherichia coli (RCMB 010052) as Gram-negative bacteria and antifungal activity against Aspergillus fumigates (RCMB 02568), Candida albicans (RCMB 05036), Syncephalastrum racemosum (RCMB 05922), and Geotricum candidum (RCMB 05097) were examined using the diffusion agar technique. Also, the cytotoxicity effect against mammalian cell lines (MCF-7cells (human breast cancer) and HTC-116 (colon carcinoma)) was evaluated. Conclusions: The obtained data proved that grafted carboxymethyl chitosan with styrene (CMCh-g-S) shows better antimicrobial activities, while parent carboxymethyl chitosan showed higher activity than new grafted materials. Also the nano scaled grafted CMCh showed higher activities than grafted carboxymetyl chitosan in macro scale.