Some new 2'[1aminomethyl{Nalkyl phthalyl benzimidazolo}]5'(2''/4''/3''/5''phenyl substituted)1',3',4'thiadiazoles have been synthesized from the Mannich base of Nalkyl phthalyl benzimidazoles (1). The Mannich base was converted to thiosemicarbazide (2) by reacting with carbon disulphide and subsequently hydrazinolysing it with hydrazine hydrate. The substituted thiosemicarbazides were then cyclized in presence of appropriate aromatic carboxylic acids under anhydrous conditions yielding the benzimidazole substituted thiadiazole derivatives (3). Chemical structures were elucidated by the spectral technique of IR, ¹HNMR and Mass. The compounds when screened for their insecticidal activity against Tribolium castaneum increased the percentage mortility of the insects.