•A kind of polyoxometalates acid is employed to prepare starch nanoparticle (SNP).•The SNP with predictable size can be prepared with high yields in short hours.•There is no change of the chemical structure of SNP.•WXRD reveals that SNP is amorphous after being treated by polyoxometalates acid.In this article, a new way of preparing starch-based nanoparticles (SNPs) with high yields by a polyoxometalates acid treatment is presented. The particle morphology, mean size and size distribution of the obtained SNPs are characterized using a dynamic light scattering (DLS) and scanning electron microscope (SEM). By changing parameters such as temperature, concentration of polyoxometalates and treating time the size of SNPs can be controlled. In addition, there are no changes in the structures of the starch granules as confirmed by IR or 1H NMR. Freeze-dried SNPs are amorphous as characterized by wide-angle X-ray diffraction (WXRD).