[Poly (N-Vinyl Pyrrollidone)] (PVP) (K30, average Mol. Wt., 40 kDa) stabilized gold nanoparticles with four different size (mean diameter) of 1.4 +/- A 0.2, 3.8 +/- A 0.4, 5.5 +/- A 0.8 and 7.8 +/- A 1 nm were synthesized using solution based chemical reduction method and used as catalysts for transfer hydrogenation reaction. Transfer hydrogenation of several organic functional groups (> C=O, -CH=O, -NO2, -CH=CH2, -Ca parts per thousand C-H, -Ca parts per thousand N) were studied using potassium formate (HCOOK) as hydrogen source in water. The major focus of this work was on catalytic activity, product selectivity and recycling ability of Au:PVP nanoparticles as a transfer hydrogenation catalyst. The size specific catalytic activity showed that up to 3.8 nm sized gold nanoparticles are highly active catalyst and afterwards its activity diminishes sharply with increasing particle size. Mechanistic insight showed that formate ions are activated on gold surface and produced bound hydride species, which is the active intermediate for this catalytic transformation.