Nanocrystalline hydroxyapatite (HAP) particles were synthesized at room temperature using reverse microemulsions, in which cyclohexane was used as the organic phase, mixed surfactant with TX-100 and 1-pentanol, and CaCl2 solution as aqueous phase. The reactor systems with aqueous/organic volumetric ratios 1: 10, 1: 5, 2: 5, and 1: 2 were carefully selected for the microemulsion processing by the pseudo-ternary phase diagram and the electric conductivity measurement of the emulsion. The as-obtained HAP nanoparticles with carbonate substitution and broadening X-ray diffraction (XRD) traces were similar to the fine powder of human bone, despite of the aqueous/organic volumetric ratio in the emulsion. No obvious other's phase occurred after as-obtained particles calcined under different temperature till 700 degrees C. In the emulsion-derived precursors, the HAP particles based on spherical morphology were prepared into the size between 15 similar to 30 nm as a low volumetric ratio of 1: 10 or 1: 5 was applied. As the volumetric ratio increased to 2: 5, the HAP particles with rod-like shape of (140 similar to 280) x (10 similar to 80) nm were formed. Practical implication of the results is that the nanocrystalline bone-like hydroxyapatite can be obtained via the emulsion processing at room temperature without further calcinations.