We undertook this research to examine the effects of P-selectin substrate density on leukocyte rolling and adhesion under linear shear flow. We have implemented a 3D immersed boundary model of a deformable cell coated with P-selectin glycoprotein-1 (PSGL-1) molecules (representing a leukocyte), rolling on a surface coated with P-selectin (representing the receptors) (Jadhav et al., 2005) subjected to shear flow. The stochastic nature of receptor-ligand bond formation and breakage was simulated using the Bell model (Bell, 1978) and the Monte Carlo method. We studied the effects of varying P-selectin density (NR) on receptor-ligand bond formation, cell rolling velocity, force per bond, and contact area between the cell and the substrate under different shear rates. We varied the NR from 30-150 molecules/µm^2 on the substrate and the shear rates from 100 to 400 s^-1. Simulations of one second of real time were conducted three times for each combination of NR and shear rate. Our results revealed that receptor-ligand bond formation increased as we increased the NR from 30 to 150 molecules/µm^2 across all shear rates. The total bond force increased as we increased the NR value. This trend was observed across all the shear rates. At the shear rate of 400 s^-1, the instantaneous rolling velocity of the cell rolling on NR= 150 molecules/µm^2 showed a smooth rolling motion of the cell compared with NR =30 molecules/µm^2. The average rolling velocity decreased from 2.6 to 2.1 mm/s (reported at 400 shear rate) with increase in the NR value. A similar trend was observed across all the shear rates. Interestingly, we also observed that although the receptor-ligand bond formation increased with an increase in NR, the percent difference in bond formation decreased. Thus, because the number of receptors present on a cell’s microvilli is fixed, the number of receptor-ligand bonds formed can reach saturation and lead the cell rolling velocity to reach a plateau.