New analysis and a definition of the new mean particle size have been discovered by rediscussing theory of the buoyancy weighing-bar method, which is a type of sedimentation methods. This paper proposes a new mean particle size based on the settling velocity in liquid. The velocity mean size of all particles ((x) over bar (0)), which is the new mean particle size defined in this paper by the buoyancy weighing-bar method, is given as follow. (x) over bar (0) = (Sigma(n)(i=1) Delta G(i)x(i)(2)/G(infinity))(1/2) (Sigma(n)(i=1) Delta G(i) = G(infinity)) Delta G(i) is the buoyancy difference mass of particles x(i). Homogenous particles and a ternary mixture of spherical glass beads are used as the samples. Although the buoyancy weighing-bar method is also time-consuming, the maximum particle size and the mean particle size can be derived within a short time after initiating measurement.