A series of 3-alkyl-5,5-dirnethylhydantoin derivatives were prepared by reacting 5,5-dimethylhydantoin with alkyl bromides with different alkyl chain lengths (C-2 to C-22). Upon chlorination, the hydration derivatives were transformed into 1-chloro-3-alkyl-5,5-dimethylhydantoins (CADMH). The structures of the samples were fully characterized with Fr-IR, H-1 NMR, UV, and DSC analyses. The antimicrobial functions of CADMH were challenged with 108-9 cfu/mL of Escherichia coli (Gram-negative bacteria) and Staphylococcus aureus (Gram-positive bacteria). All the samples provided a total kill of the Gram-negative and Gram-positive bacteria in less than 30 min. CADMH were used as antimicrobial additives for polymeric materials. It was found that the presence of as low as 1% of CADMH could provide the samples with potent antimicrobial functions. The structure-anti microbial efficacy relationships of the CADMH are further discussed.