An apparatus for the measurement of particle velocity and the diameter of a spherical particle (smaller than 10 mm) using acoustic emission due to the impact of the particle on a circular plate is developed. Based on the dropping experiments of nylon, high carbon chromium bearing steel (SUJ) and glass particles, the initial peak height (P₁) and the peak frequency (f_p) of the impact sound are related to the impact velocity (v), particle diameter (D), Young's modulus (E_p), Poisson's ratio (μ_p) and particle density (ρ_p). The P₁ and f_p are empirically expressed by Eqs.(6) and (15) respectively. If the properties of spherical particle are known, it is possible to estimate D and v from these equations by measuring P₁and f_p of the acoustic signal due to the impact of a particle. If a mass of spherical particle is known, it is also possible to estimate E_p and μ_p from these equations by measuring D and v using the apparatus.