Synthesis of new derivatives of pyrido[2,3-d][1,2,4]triazolo[4,3-a]pyrimidin-5-one via reaction of 7-(4-bromophenyl)-1,2-dihydro-5-(4-fluorophenyl)-2-thioxopyrido[2,3-d] pyrimidin-4(3H)-one with hydrazonoyl chlorides or reaction of 2-hydrazino-pyrido[2,3-d]pyrimidin-4(3H)-one with different aldehydes followed by cyclization of the products. All the newly synthesized compounds were evaluated for their antimicrobial activities and also their minimum inhibitory concentration against most of test organisms was performed. Amongst the tested compounds displayed excellent activity against all the tested microorganisms except SR and PA.