Novel series 2-(1-substitutedguanidino-3-yl)-4-(3-phenylthiocarbamido-1-yl)-6- substituted imino-1,3,5-thiadiazine [3a(i) to 3f(ii)] have been obtained by basification of their hydrochlorides [2a(i) to 2f(ii)] in presence of ammonium hydroxide solution, which are synthesized by the interaction of 1-Formamidino- (n- substitutedthioamido)-5-phenyl-2-thio-4-iminobiuret (1a-f) and N-aryl/alkylisocyanodichlorides. The latter were prepared initially by the condensation of substitutedisothiocyanate with N-phenylformamidi-noformamidinothiocarbamide. The structure of all these compounds were established on the basis of elemental analysis, IR and PMR spectral data. All the synthesized compounds have been assayed for their antimicrobial activity against both gram-positive and gram-negative human pathogens and found that they possess insecticidal, and bacteriocidal.  Some 1,3,5-thiadiazine compounds show remarkable biological activity.