A new equation for estimating hindered settling velocity of a particle in a polydisperse suspension (in which particle sizes are distributed continuously) was derived by taking into account particle size distribution and particle concentration of the suspension. First, a basic equation of the settling velocity of a particle was proposed on the basis of theoretical consideration of the relation of particle concentration and particle size distribution to drag force and the velocity of fluid flow induced by settling of particles. To determine the unknown parameters included in the basic equation, the influences of particle concentration, the volume fraction of particles and particle size ratio on the settling velocity of particles of two different sizes were investigated by computer simulation using a discrete element method and direct numerical simulation. The influences of fluid velocity and drag force on the settling velocity of particles were represented as a function of particle concentration and particle size distribution, and then the new equation of hindered settling velocity was derived. The estimated values of hindered settling velocity of an individual particle in a polydisperse suspension of particles with continuous particle size distribution agreed well with the experimental values.