This study has investigated theoretically longitudinal dispersion of suspended fine particles with settling velocity and revealed the dispersing behavior from the initial to the stationary stages in tidal oscillatory currents. A Stokes layer of the oscillatory current plays a remarkably significant role on dispersion of settling particles when the mixing time in cross-section is larger than the period of tidal oscillation. When the settling velocity exceeds a certain value, however, the dispersion coefficient of particles decreases with increasing settling velocity. In an oscillatory current with linear profile, settling velocity of dispersing particles decreases the value of the dispersion coefficient all over the values of the settling velocity.