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The dynamic contact behavior of hydrodynamic journal bearings during start-up is investigated. The hydrodynamic oil force and asperity contact force are obtained by solving the mixed lubrication model. The motion of the journal center, the contact time, and the lift-off speed of the bearing are determined. The effects of the relative clearance and acceleration time are discussed. The result shows that the hydrodynamic oil force increases sharply during the early stage of the start-up process, leading to a sharp decrease in the contact force. An increase in the relative clearance of the bearing generates a decrease in both the contact force and the contact time. Also, a high start-up acceleration leads to a sharp decrease in the contact force. [ABSTRACT FROM AUTHOR]