In a search for potential antimicrobial compounds thirteen new 3-(1-phenylethyl)-5substituted-tetrahydro-2H-1,3,5-thiadiazine-2-thiones were synthesized by the reaction of  phenethylamine with carbon disulfide and potassium hydroxide, followed by formaldehyde and the appropriate alkyl, aralkylamines, glycine or ethyl glycinate (Scheme 1). The chemical structure of the synthesized compounds was elucidated by spectral data and elemental analysis. The title compounds were tested, in vitro, for antimicrobial activity against Gram-positive, Gram-negative bacteria, and some fungi, using agar disc method. The antimicrobial activity was found to be affected by the bulkiness of the side chain and presence of polar group at N5 position. The highest activity was obtained with compounds 4l and 4m (R= CH2-COOH, CH2COOC2H5).