In this study, a novel series of heterocyclic compounds containing pyrazoline nucleus has been synthesized. The compounds were synthesized in two steps. Chalcone was synthesized in the first step by Claisen-Schmidt reaction, using 1-acetylnaphthalene and p-nitro benzaldehyde as reactants. In the second step, the chalcone was cyclized in an acidic medium with some hydrazine derivatives to form pyrazolines. All the compounds were characterized by physical, chromatographic, spectroscopic, and elemental analysis and evaluated in vitro for antimicrobial activity against nine microorganisms by cupplate method. The minimum inhibitory concentration of all the compounds was determined by tube dilution method. All the compounds exhibited higher antibacterial activity as compared to the antifungal activity. Compound 5g (3-Naphthalen-1-yl-1-(2-nitro-phenyl)-5-(4-nitro-phenyl)- 4,5-dihydro-1H-pyrazole) exhibited maximum antibacterial and antifungal activity and may be designated as the most potent member of the series, with 2-nitrophenyl group at N-1 position of the 2-pyrazoline ring.