Starch nanoparticles (SNPs) were prepared for the first time via a combination of ultrasonic-assisted dissolution of starch and subsequent rapid nanoprecipitation. Purified starches from cereal (regular corn and wheat) and pulse (faba bean and field pea) grains that significantly differ in amylose content were used. The effects of starch source, concentration and amylose content as well as the ratio between antisolvent to solvent (i.e., aqueous starch solution) on the SNPs morphological, molecular, and physicochemical properties were investigated. The morphology was evaluated by scanning electron microscopy (SEM) to observe shape and surface details and dynamic light scattering (DLS) to analyze particle size and poly dispersity index (PDI). Molecular properties of the native starch and SNPs were determined by a high-performance size-exclusion chromatography equipped with a multi angle laser light scattering and a differential refractive detector (HPSEC-MALLS-RI). Physicochemical characterization of the SNPs was performed via measuring the crystallinity by X-ray diffraction (XRD), molecular order by Fourier-transform infrared spectroscopy (FTIR), and thermal properties by differential scanning colorimeter (DSC). Results showed that SNPs from all starches were spherical in shape, where pulse SNPs had smaller and more uniform size than cereal SNPs. Across all starch types, the molecular weight of amylopectin and amylose in SNPs was smaller and more uniform than in the respective native starches. Pulse SNPs with higher amylose content showed greater relative crystallinity, enhanced short-range molecular order, and better thermal stability. A schematic diagram has been proposed to explain the variations in SNP size, where amylose-amylopectin ratio plays an important role.