The strategy of impregnating lignocellulosic biomass with aqueous metal salts solutions was efficiently applied in our group for producing H2-rich gas by pyrolysis of wood at 700 °C. The present work is a thorough study on the formation of nickel metal nanoparticles (NPs) during the pyrolysis of nickel(II) impregnated wood in the temperature range of 400-500 °C. During the wood impregnation step, the numerous oxygenated groups present in the bio-macromolecules act as adsorption sites for metal cations in aqueous medium yielding very high metal precursor dispersion into the wood matrix. During pyrolysis, an amorphous NixOyHz phase is formed which is reduced to metallic nickel (Ni0) by carbon atoms at temperatures below 500 °C, leading to the formation of Ni0 NPs, as demonstrated by XRD, XPS, STEM and temperature-programmed pyrolysis characterizations. These findings reveal that the in situ formed Ni0 nanocrystallites act as the catalytic active phase for enhancing both H2 production and tar conversion during biomass pyrolysis, i.e. during the initial stage of gasification. Potential interests of these in situ generated Ni nanocatalysts for biomass gasification applications are discussed.