Abstract: To better understand genetic control of physical properties of starch granules, we attempted a new approach to analyze developmental and genotypic effects on morphology and size distributions of starch granules in sweetpotato storage roots. Starch granules in sweetpotatoes exhibited low sphericity, many shapes that appeared to be independent of genotypes or developmental stages, and non-randomly distributed sizes. Granule size distributions of sweetpotato starches were determined in high resolution as differential volume-percentage distributions of volume-equivalent spherical diameters, rigorously curve-fitted to be lognormal, and specified using their geometric means x¯∗ and multiplicative standard deviations s∗ in a x¯∗×/(multiply/divide)s∗ form. The scale (x¯∗) and shape (s¯∗) of these distributions were independently variable, ranging from 14.02 to 19.36 μm and 1.403 to 1.567, respectively, among 22 cultivars/clones. The shape (s∗) of granule lognormal volume-size distributions of sweetpotato starch were found to be highly significantly and inversely correlated with their apparent amylose contents. More importantly, granule lognormal volume-size distributions of starches in developing sweetpotatoes displayed the same self-preserving kinetics, i.e., preserving the shape but shifting upward the scale, as those of particles undergoing agglomeration, which strongly indicated involvement of agglomeration in the formation and growth of starch granules. Furthermore, QTL analysis of a segregating null allele at one of three homoeologous starch synthase II loci in a reciprocal-cross population, which was identified through profiling starch granule-bound proteins in sweetpotatoes of diverse genotypes, showed that the locus is a QTL modulating the scale of granule volume-size distributions of starch in sweetpotatoes.