This paper presents the effect of preload on nonlinear dynamic behavior of a rigid rotor supported by two-lobe aerodynamic noncircular journal bearing. A finite element method is employed to solve the Reynolds equation in static and dynamical states and the dynamical equations are solved using Runge-Kutta method. To analyze the behavior of the rotor center in the horizontal and vertical directions under different operating conditions, the dynamic trajectory, the power spectra, the Poincare maps and the bifurcation diagrams are used. From this study, results show how the complex dynamic behavior of this type of system comprising periodic, -periodic and quasi-periodic responses of the rotor center varies with changes in preload values by considering two bearing aspect ratio. The results of this study contribute to a better understanding of the nonlinear dynamics of two-lobe aerodynamic noncircular journal bearing system.