The effect of bearing length to diameter (L/D) ratio and large disk position on nonlinear vibration of a flexible rotor-bearing system was investigated. The rotor consisted of a shaft modeled by 1-D finite elements and disks. It was supported by a self-aligning ball bearing and an axial-groove journal bearing. Two journal bearing's L/D ratios of 0.4 and 0.6, two large disk positions: 340 and 575 mm measured from the ball bearing, and two bearing models: considers both journal's lateral and angular motion (Model A) and considers only journal's lateral motion (Model B), were investigated. The degrees of freedom (DOF) of the equation of motion (EOM) were reduced to those of the boundary DOF by real mode component mode synthesis that retains only the first forward and backward modes of the internal DOF. Shooting method and Floquet multiplier analysis were applied to the reduced EOM to obtain limit cycles and their stability, which indicates Hopf bifurcation type. Numerical results indicated that supercritical bifurcation only occurred in the case of L/D=0.4 and large disk position 575 mm for both bearing models. Otherwise, the subcritical bifurcation occurred except the case of L/D=0.6 with the large disk position 575 mm that supercritical bifurcation occurred if Model B was used. The experiment with the same parameters used in the calculation was conducted as verification. The experimental results showed the same bifurcation type as calculated by using Model A.