Background and Objectives Fundamentally it is important to understand starch granule initiation and deposition of starch molecules during the growth of granules. The objective of this study was to investigate the morphology, composition, and structure of starches in sorghum from fifth day post‐anthesis (DPA) until the maturity (25 DPA). Findings The minimal size of sorghum starch for showing Maltese cross was 4 µm. The average size of starches on 5 DPA was 3.2 µm, and most starches did not exhibit Maltese cross. Amylose content was low (13.0%) on 5 DPA and increased to 31% on 25 DPA. The size of amylose was long with a peak at DP 1771 on 5 DPA and changed during the starch biosynthesis. The short‐chain amylopectin proportion significantly increased on 25 DPA. Conclusions The low amylose content and high proportion of long-chain amylose might be favorable for the initial sorghum starch formation. The starch polymers were less radially oriented in the primary starches. Amylopectin in the periphery of a large sorghum starch was more branched than that of the inner part. Significance and Novelty The variations in the amylose length distributions and orientation of starch polymers provide new information on starch biosynthesis.