An efficient, mechanochemical reaction for synthesizing ZnO nanoparticles has been investigated. The starting materials were zinc sulfate heptahydrate and potassium hydroxide, and potassium chloride served as the matrix salt. The reaction was carried out in a paste state at room temperature with a short grinding time and no postreaction calcination. The nanoparticles thus obtained had a mean diameter of 22.1 nm and exhibited excellent UV-blocking properties. A comparison of the costs of raw materials between this and other mechanochemical reactions is presented.