Nonlinear behavior of a multirotor system supported by oil film bearings is investigated theoretically by calculating the loci of centers of disks and a journal of the system using a time-marching method. The oil film force of bearings is obtained by solving the Reynolds equation at each time step using a rapid solution presented previously by the authors. It will be shown that a 2-disk/4-bearing rotating system operated under the linear stability limit but above twice the first critical speed becomes unstable when sufficiently large external shocks-an artificial sinusoidal shock and real seismic waves (Taft, California, 1952)-are given to the system. The effect of misalignment of bearings is also examined closely in this study.