Uniform nanorods of hydroxyapatite (HAP) with an unusual orthorhombic shape have been synthesized from homogeneous solutions of Ca2+ and HPO42– in the presence of gelatin and urea. The lengths of the nanorods are in the range of hundreds of micrometers, and the widths are about 100 nm. The HAP phase is generated by the transformation from its precursor phase of octacalcium phosphate (OCP), which has been monitored by X-ray diffraction, NMR spectroscopy, scanning electron microscopy, and transmission electron microscopy. The rise in pH due to the decomposition of urea drives the OCP transformation to HAP. In the presence of gelatin, nanorods of OCP phase formed first and then transformed into the HAP phase, preserving the single-crystal morphology. On the other hand, blade-like OCP crystals form from the solution in the absence of gelatin. On increasing the pH of the solution, the large, blade-like OCP crystals tend to crash into irregular, hexagonal HAP crystallites. A single-crystal-to-single-crystal topochemical transformation may be attributed to the evolution of HAP nanorods from the precursor OCP phase. This gives a strong indication as to the OCP to HAP transformation mechanism in the mineralization of biological apatite in tooth enamel and bone.