GEMS-0067 maize starch contains up to 32% elongated starch granules, much higher than amylose-extender (ae) single-mutant maize starch (∼7%) and normal (non-mutant) maize starch (0%). These elongated granules are highly resistant to enzymatic hydrolysis at 95–100 °C, which function as resistant starch. The structure and formation of these elongated starch granules, however, were not known. In this study, light, confocal laser-scanning, scanning electron, and transmission electron microscopy were used to reveal the structure and formation of these elongated starch granules. The transmission electron micrographs showed fusion through amylose interaction between adjacent small granules in the amyloplast at the early stage of granule development. A mechanistic model for the formation of elongated starch granules is proposed.