The present work consists of four parts. The first part deals with the synthesis of new series 1,3,5-tri-[5(4'-n-alkoxyphenyl)-1,3,4-thiadiazol-2-yl-]-trimesilydene [7]n, involving four steps which were outlined as follows: Step one: Preparation of 4-n-alkoxybenzoic acid[2]n The reaction of 4-hydroxybenzoic [1] acid with different n-alkyl halides afforded the corresponding 4-n-alkoxybenzoic acid[2]n. Step two: Synthesis of 2-amino-5-(4-n-alkoxyphenyl)-1,3,4-thiadiazole[3]n The reaction of thiosemicarbazide and different 4-n-alkoxy benzoic acid[2]n, in POCl3 afforded the corresponding 2-amino-5-(4-n-alkoxyphenyl)-1,3,4-thiadiazole[3]n. Step three: Preparation of 1,3,5-triformylbenzene[6] First, step 3A the reaction of mesitylene with bromine under a source of UV light afforded -hexabromomesitylene[5]. Second, step 3B the reaction of "-hexabromomesitylene[5] with morpholine and concentrated hydrochloric acid afforded 1,3,5-triformyl benzene[6]. Step four: Synthesis of 1,3,5-tri-[5(4'-n-alkoxyphenyl)-1,3,4-thiadiazol-2-yl-]-trimesilydene[7]n The reaction of 1,3,5-triformylbenzene [6] with different 2-amino-5-(4-n-alkoxyphenyl)-1,3,4-thiadiazole[3]n afforded the titled compounds [7]n . Part two Characterization: The characterization of the synthesized compounds was elucidated by different spectroscopic methods e.g., (UV, IR,) spectroscopy. The results are in agreement with the suggested structures. Part three Liquid crystals: The liquid crystalline behavior of the Schiff's bases containing 1,3,4-thiadiazole[7]n ring system have been examined by means of hot-stage microscopy. Texture observations suggested the existence of LC phases for the compounds [7]4, [7]5,[7]6,[7]7, and [7]8 while compounds [7]1, [7]2, [7]3 do not form liquid crystals phases. Part four Biological activity: The synthesized compounds were tested in vitro for antimicrobial activity. The results obtained indicated that some of these compounds are more active than the others. Table (3-5).