Wet cleaning of deep trenches, 4 ~m deep and 0.5 ~m wide, was studied using a two-dimensional wet cleaning simulation including both flow and diffusion effects. It has turned out that the diffusion effect is a dominant factor during wet cleaning. When the flow velocity of the rinse water increases, the rate at which solution in trenches is replaced becomes higher. In our simulation, the solution in the deep trench is diluted to 1 ppm by rinse water within several seconds. This result shows that deep trenches with submicron width can be cleaned by a wet cleaning process. Uniform etch rates of oxide in deep trenches suggest the diffusion mechanism is predominant.