Nickel nanoparticles (Ni NPs) with a crystalline size of around 30 nm have been synthesized successfully via the chemical reduction method. Ni NPs were obtained through a nickel salt with hydrazine hydrate at 80 °C temperature by using ethylenediamine as protective agent. The synthesized nanoparticles were characterized by using FTIR spectroscopy, powder X-ray diffraction pattern, ultraviolet-visible spectroscopy, energy dispersive X-ray spectroscopy (EDS), thermogravimetry (TG/DTG), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The size and morphology behavior of NPs were studied by PXRD, SEM and TEM techniques. Furthermore, its applications studies were carried out as catalyst for Knoevenagel condensation reaction of aromatic aldehydes and malononitrile under solvent free conditions. The efficacy of NPs catalyst was exhibited an excellent recyclability and reusability up to four times without any additional treatment. The silent feature of nickel nanoparticles were found as efficient, cleaner reactions profiles and simple workup. Moreover, its comparative antibacterial activities were performed by using common solvents and sonication under standard method. The antibacterial activities were tested against human bacterial pathogen such as Staphylococcus aureus, Escherichia coli, Klebsiella sp, Enterococcus faecalis and Pseudomonas aeruginosa using well diffusion method. Nonetheless, the antibacterial activities of Ni nanoparticles (20 to 60 ug) were compared with four well known antibiotics i.e. Amikacin (30 mcg), Ciprofloxacin (5 mcg), Gentamicin (5 mcg) and Norfloxacin (10 mcg). The highest antimicrobial activity of Nickel nanoparticles were found against Pseudomonas aeruginosa, Staphylococcus aureus (21 mm) and Klebsiella sp. (20 mm). However, the results reveal an efficient antimicrobial activity against pathogenic bacteria under sonication than common solvent technique.