In thisJournal Bearing Rotors chapter, the mathematical model of a four-degree of freedom (4-DOF) journal bearing rotor system is studied. Period-2 motions in such a system are obtained through the semi-analytical methodSemi-analytical method. The vibration of disk and bearing case varying with excitation frequency is studied, and excitation effects on journal baring rotor system are discussed. Numerical simulationsNumerical simulations of period-2 motions are completed. The phase trajectories, time-histories of displacements are presented. The harmonic amplitudes and harmonic phases varying with harmonic orders are presented.