A two-dimensional (2D) computational model for simulation of incipient sediment motion for non-cohesive uniform and non-uniform particles on a horizontal bed was developed using the Discrete Element Method (DEM). The model was calibrated and verified using various experimental data reported in the literature and compared with different theories of incipient particle motion. Sensitivity analysis was done and the effects of relevant parameters were determined. In addition to hydrodynamic forces such as drag, shear lift and Magnus force, the particle-particle interaction effects were included in the model. The asymptotic critical mobility number was evaluated for various critical particle Reynolds numbers (R*) in the range of very small and very large R*. The obtained curve is classified into four regions. It was found that in the linear region, the drag force has the principal role on the initiation of motion. Moreover, the critical mobility number is independent of particle diameter. A procedure for estimating the critical shear velocity directly from the information on particle diameter and roughness height was developed. Finally, the mechanism of incipient motion for the different regions was studied and the effect of different forces on the incipient particle motion was obtained. It was found that the maximum effects of lift and Magnus forces were, respectively, less than ten and twenty percent of the total force. The drag force, however, was typically the dominant force accounting for majority of the net hydrodynamic force acting on sediment particles at the onset of incipient motion.