We conducted numerical experiments on nonlinear tide and fine sediment transport in the Ariake Sea and the Yatsushiro Sea by Delft3D model. As a result of the present research the followings are clarified: i) Amplitude of S2 tide correlates inversely with M2 tide; ii) Maximum velocity of nonlinear tidal current is around 20cm/s near bay mouth area of both bays; iii) In the case excluding tidal flat sedimentation is enhanced; and iv) Variation of nodal factor f for M2 tide can change erosion/sedimentation characteristics around mouth of Isahaya Bay.