A microfluidic device using droplet-fused microreactors is introduced for room temperature synthesis of nanoscale needle-shaped hydroxyapatite (HAp, Ca10(PO4)6(OH)2). The device is integrated with multifunctional units, e.g., T-junctions for droplet generation and fusion, winding channels for rapid mixing, and a delay line for simple visualization of the HAp formation process. The necessary conditions such as surfactant and fluid flow rate for an aqueous stream to merge with water-in-oil droplets are investigated. The nanoscale morphologies of the HAp produced by this method are also compared with HAp prepared by conventional bulk mixing. This paper shows that further reaction could be initiated by flowing additional reagent streams directly into the droplets of the initial reaction mixture, which is a novel approach for synthesizing a needle-like morphology of the HAp with a high aspect ratio under room temperature.