Nickel(II) complexes of four new aroylhydrazones,N-(2-hydroxy-1-naphthaldehyde)- N' -(4-nitrobenzoyl)hydrazone(H2L1), N-(2-hydroxy-4-methoxybenzaldehyde)-N'-(2-furoyl) hydrazone (H2L2), N-(2-hydroxyacetophenone)-N' -(2-theonyl) hydrazone (H2L3) and N-(2-hydroxy-5-methylacetophenone)- N' -(3-toloyl) hydrazone (H2L4) have been synthesized. The structure of these complexes has been investigated using elemental analysis, magnetic susceptibility, molar conductance, thermogravimetric analysis and spectral (IR, electronic, NMR) measurements. IR spectra indicates that the ligand behave as dibasic tridentate ligand and coordinate to Ni(II) ion through ketonic oxygen, azomethine nitrogen and phenolic oxygen atom (except in H2L1 by naphtholicoxygen atom) by double deprotonation.TG curve show decomposition of the complexes with NiO being the final decomposition productconfirmed by X-ray data. The magnetic and spectral data suggest the octahedral geometry for all the Ni (II) complexes. Antibacterial activity of the ligand and their Ni(II) complexeswere studied against both Gram-positive (staphylococcus aureus) and Gram-negative (Escherichia coli, Klebssiella pneumoniae) bacteria using minimum inhibitoryconcentrations (MICs) method.