Abstract: Based on the analysis of some equivalent diameters of non-spherical particles, both the Stokes diameter and Newton diameter are recommended as the characteristic dimensions of non-spherical particles settling in the intermediate region. Through the use of Kaskas correlation on drag coefficient, a new equation for estimating the terminal falling velocity has been derived as following: The equation is applicabe to both Stokes Law region and Newtons law region. The characteristic dimensions were dependent upon the size distribution. The data from sieve analysis were processed with cubic spline function interpolation, and two equations for estimating the nominal diameter were derived for the particles settling in Stokes law region or Newtons law region respectively. The nominal diameter may be modified by shape coefficient to obtain the Stokes diameter and Newton diameter. A series of experiments carried out in a fluidized bed equipment showed that the calculated values fitted well the experimental data.