A three-dimensional direct simulation of an immersed solid particle approaching another particle, or a flat wall, is conducted to investigate the mechanics of hydrodynamic impact of immersed particles. The simulation method is based on a modified immersed boundary method using a fixed grid system. When the particle separation distance be- comes smaller than grid spacing, to account for the hydrodynamic resistance effect of liquid layer between particles near contact, a microlayer model is developed to allow determination of the pressure profile within the micro-layer without neglecting the inertial force of the layer flow. The pressure force is then taken into account in equation of particle motion. Comparisons of the simulation results with the experimental results reported in the literature are shown to substantiate the model presented in this study. The simulations reveal the complex three-dimensional flow field of the liquid and the motion of the approaching particle. The fluid pressure in the gap caused by the unsteady motion of the particle is sig- nificantly increased when the separation distance of particles is less than about one-tenth diameters of particle. Therefore the velocity of approaching particle starts to decrease due to the hydrodynamic resistance force at this position. A three-dimensional direct simulation of an immersed solid particle approaching another particle, or a flat wall, is conducted to investigate the mechanics of hydrodynamic impact of immersed particles. The simulation method is based on a modified immersed boundary method using a fixed grid system. When the particle separation distance be- comes smaller than grid spacing, to account for the hydrodynamic resistance effect of liquid layer between particles near contact, a microlayer model is developed to allow determination of the pressure profile within the micro-layer without neglecting the inertial force of the layer flow. The pressure force is then taken into account in equation of particle motion. Comparisons of the simulation results with the experimental results reported in the literature are shown substantiate the model presented in this study. The simulations reveal the complex three- dimensional flow field of the liquid and the motion of the approaching particle. The fluid pre ssure in the gap caused by the unsteady motion of the particle is sig- nificantly increased when the separation distance of particles is less than about one-tenth diameters of particle. Thus the velocity of approaching particle starts to decrease due to the hydrodynamic resistance force at this position.