Flow behavior of particles discharged from an orifice of a hopper is simulated three dimensionally by the Particle Element Method, PEM (Discrete Element Method, DEM), and the relation between the mass flow rate (W) and the orifice diameter (D) is discussed in conjunction with the cyclic formation-collapse phenomena of the arch formed over the orifice. The simulated relation is consistent with the empirical one, expressed as W proportional to D-n (n = 2.5-3.0). W is expressed as functions of three parameters, which are the cross sectional area of the orifice (A), the flow velocity (v), and the volume fraction of flowing particles at the orifice (gamma). A is the parameter with most effection W, because it is proportional to D-2. In addition, v influences W at one-quater of A＇s contribution, while gamma＇s contribution is only one-quater of A＇s one or less, depending on the frictional coefficient of particle. The frictional coefficient of the particle is an important factor for controlling the mass flow rate of particles.