We propose a Navier-Stokes type slip boundary condition in the contact line dynamics. To validate our model, a water droplet, whose volume is on the order of 10 microliters, sitting on a horizontal or inclined plane has been studied both experimentally and numerically. Different droplet sizes with plane tilted angles and two substrates (Teflon and copper) have been investigated. Our model predicts the equilibrium, advancing and receding contact angles consistent to the experimental results. When the substrate is inclined before reaching a critical angle, we observe the contact angle hysteresis. An adhesive tension, equivalent to the concept of the line tension, at the contact line is applied to predict (1) the contact angle hysteresis, and (2) the critical tilted angle, when the droplet starts to slide down. For each substrate surface, two maximum adhesive tensions at the advancing and receding sides of the contact line are computed based on the comparisons between simulations and experiments. Results show that the same maximum adhesive tensions are captured for all four droplet sizes on the same substrate surface.