We introduced a nanometer-sized reaction pool in the hydrothermal synthesis of ceria (CeO2). The addition of a heatstable biopolymer, lignosulfate, in the hydrothermal synthesis resulted in the drastic downsizing of CeO2 particle to 5 nm. We, further, show that the lignosulfate functions as an accelerator for the CeO2 nanoparticle synthesis.