In this paper, monodisperse Pd nanocrystals were immobilized on previously reported carbon nanospheres via in situ adsorption and reduction. In this protocol, no excess reductant and capping reagents were necessary, which made the surface of the as-prepared nanocatalysts very clean. Using sodium tetrachloropalladate(II) as the metal precursor yielded palladium nanocrystals with a size around 5 nm regardless of the metal loading, while the use of palladium chloride resulted in a size increase to 18.1 nm. Moreover, the additives used during preparation have been proven to be of great importance in controlling the average particle size. It was suggested that the pattern of the adsorbed palladium ions or the surface environment of support was greatly influenced. Strong adsorption of the palladium ions on the carbon spheres led to a decrease in size. The nanocrystals exhibited excellent catalytic activity for transfer hydrogenation under ambient conditions. The conversion was 83.1% to 100% for several nitroaromatics with moderate to excellent selectivity. More importantly, these nanocatalysts are promising for renewable catalysis owing to their sustainable support, green catalyst fabrication and ease of handling.