The synthesis of substituted pyrazolo [3',4':4,5]thieno [2,3-d]pyrimidin-8-ones (IIIa-j) from 5-amino-3-methyl-1H-thieno[3,2-c]pyrazole-6-carbonitrile (II) is described. The key compound II was synthesized from (5-methyl--2,4-dihydro-3H-pyrazol-3-ylidene)malononitrile I via the Gewald reaction. The synthesis of the title compounds IIIa-j was accomplished by condensation of II with different aromatic aldehydes. The newly synthesized heterocyles were characterized by elemental analysis, IR, 1 H-NMR, 13 C-NMR and mass spectroscopic investigation. All the newly synthesized compounds were evaluated for antimicrobial activity against a variety of bacterial strains.