Abstract: Ten 3-benzyl-5-subetitued tetrahydro-2H-1,3,5-thiadiazine-2-thiones weresynthesized and seven of them are reported for the first time.The structures of the compounds havebeen elucidated by UV,IR,H-NMR and elemental analysis.The in vitro activity of the compoundsagainst 6 kinds of bacteria and 2 kinds of fungi was tested.The antimicrobial activities of allcompounds are more potent than sulfadiazine sedium and less potent than norfloxacini.All compoundswere found to be more active against gram-negative and less active against gram-positive bacteria andweak against fungi.