We report a comparative structural and magnetic study of nickel oxide (NiO) nanoparticles and NiO/Ni3B2O6 nanocomposites synthesized by chemical reduction of Ni2+ ions with BH -4 in aqueous solution, under ambient atmosphere and subsequent suitable annealing. The structure and magnetic properties have been carried out to understand the chemical mechanism of this reduction method. Our experimental results show that this nanocomposite displays an anti ferromagnetic phase transition at Neel temperature (TN) ∼48 K, which differs from super paramagnetism in NiO nanoparticles. This study shows that relative molarities of the precursor and reductor also play important role in obtaining the final product in FM, AFM or superparamagnetic state. © 2008 American Institute of Physics.