Abstract: The computational fluid dynamics is used to investigate the flow field inside the real root canal during the irrigation, to evaluate the irrigation efficiency, as well as the effects of the irrigating fluid velocity and the working length on the flow pattern, the velocity, the shear stress and the apical pressure of the irrigating fluid. The real human root canal model and a 30 g flat head needle are adopted in the numerical simulation. It is shown that increasing the inflow velocity can not significantly improve the irrigation efficiency, but the risk of the apical extrusion is increased greatly because of the increase of the apical pressure. The working length can not always improve the displacement ability. An appropriate working length can be adopted to obtain the best displacement, together with the low apical pressure. Excessive increase of the working length might decrease the displacement and the shear stress on the wall, with a reduced irrigation efficiency.