We aimed to develop a diabetes mellitus (DM) treatment that could be administered by intramuscular (IM) injection and it lasted for more than 3 days. The objective was to load insulin into the lattice space of hydroxyapatite (HAP) to prevent its release based on a concentration gradient or detachment from the surface, with insulin release being aided by cellular activity. To avoid insulin denaturation during the synthesis of insulin-loaded HAP (insHAP), we developed a single-step insHAP synthesis method by the hydrolysis of brushite. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) results suggested that insulin could be loaded into the HAP crystal lattice. After IM administration in rats with DM, the synthesized insHAP is thought to be engulfed by macrophages, escape from lysosome/endosome hybrids following disruption by osmotic pressure, and pumped into the extracellular space before entering the blood stream by diffusion. In rats with DM, the normal blood glucose level was maintained for 4 days after a single IM injection of the synthesized insHAP particles. Thus, insHAP may provide a breakthrough in insulin delivery for DM treatment, and may also be used to deliver fluorescent proteins, antibodies, and anticancer drugs.