Semicrystalline polyethylene (PE) nanoparticles were obtained by free radical emulsion polymerization under mild conditions of pressure and temperature. At high surfactant concentration, the obtained anisotropic nanoparticles exhibit a strong degree of supercooling. This paper explores the relationship between the morphology of such particles and their high degree of supercooling. The shape anisotropy of the semicrystalline PE particles already observed by transmission electron microscopy was confirmed by dynamic light scattering with the autocorrelation function containing both translational and rotational diffusion coefficients as well as by in situ synchrotron small-angle and wide-angle X-ray scattering (SAXS/WAXS). Temperature-resolved synchrotron scattering was used to further characterize the evolution of particles morphology and crystallinity during cooling. Paying special attention to the role of the surfactant, we propose a novel mechanism which contributes to a better understanding of the crysta...