In this research, synthesis of lanthanum oxide nanoparticles using supercritical water as a reaction medium in batch type reactor was studied. The crystallographic identity and morphology of the synthesized nanoparticles were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns indicate that the well-crystallized lanthanum oxide nanocrystals can be easily obtained under the current synthetic conditions. The effect of four parameters includes temperature, reaction time; primary concentration of aqueous solution of lanthanum (III) nitrate and pH of starting solution on reaction efficiency, particle size and the BET surface area were investigated using 2(4-1) fractional factorial design. Finally, by employing a regression analysis a model based on effect of significant main variables and their binary interactions was proposed which can predict the percentage of reaction efficiency with acceptable confidence. (C) 2010 Elsevier B.V. All rights reserved.