Grain size distributions analyzed by a laser diffraction-scattering method have been compared with those by a conventional hydrometer method. The laser method shows coarser shifts of grain size distribution for all samples. For the reason, it is pointed out that the difference of the physical meaning of the grain size measured by these two methods, and the effects of hindered settling, turbulence of the liquid and Brownian motions to the Stokes velocity for the hydrometer method.