Ca(OH)2 nanoparticles have been synthesized with high purity and yield using the hydrogen plasma-metal reaction method. They are spherical in shape with a mean particle size of approximately 100 nm. The morphology of nanoparticles is spongy with mesopores, mostly less than 10 nm. The pore volume and surface area of Ca(OH)2 nanoparticles are 0.084 cm3/g and 28.7 m2/g, respectively. Both transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis demonstrated that these nanoparticles possess poly-nanocrystalline structure with an average grain size of about 10 nm. The formation mechanism of Ca(OH)2 nanoparticles was discussed in terms of chemical reactions and coalescence during the processing.