Choosing the appropriate end supports has been great importance in rotating machinery. The mechanical bearings such as ball and journal bearings are more popular types of supports that used in rotating systems. However, the rub- impact between the rotor and bearing is main disadvantage of these bearings, that results in nonlinear behaviors. Whereby, the magnetic bearings have been developed recently that overcomes the previous problems, but induces a new nonlinear factors that affect the dynamical behavior of systems. In this paper, the chaotic behavior of a flexible rotor supported by active magnetic bearings under the influence of nonlinear forces due to the electromagnetic field of bearing force and the unbalancing force is investigated. The rotor-Active Magnetic Bearing (AMBs) systems include many non-linear factors, such as nonlinear function of the coil current and the air gap between the rotor and the stator, nonlinearity due to geometric coupling of magnetic actuator, eddy current effect, flux leakage and hysteresis losses of the magnetic core material. A statically unbalanced disk is mounted on the mid span of the shaft. The rotor is modeled as three masses and 8 degree of freedom that is supported by two magnetic bearings with four- polar coils. The shaft divided to two lumped masses in two ends of shaft. Therefore they are allowed only planer displacements in x and y, while the rigid disk is allowed both x and y displacements plus x and y angular displacements. So the gyroscopic moments of disk is impressed in the equations. With this model, two flexible half shaft can be either treated as massless or subdivided into lumped masses that are combined with concentrated mass of disk. The equations of motion are extracted under Euler-Bernoulli beam theory. The governing dynamics equations of the system are extracted in form of eight nonlinear coupled second order ordinary differential equations by the Lagrange’s equations. These equations have solved by variable step solver based on the fourth order Runge-Kutta (ODE 45) method. The main point in obtaining reliable results is to select proper time steps for the numerical integration. The influence of rotor speed, shaft flexibility, magnetic bearing stiffness, weight parameter, external damping, proportional and derivative gains of controller are investigated on the behavior of system. The bifurcation diagrams, time history, phase plane portrait, power spectra, Poincare map and Maximum Lyapunov exponents are used to analyze the response of the system under different operational conditions. The numerical result shows a rich variety of nonlinear dynamical behavior including periodic, sub-periodic 2, 3, 4, 6, 12, quasi periodic and chaotic vibrations due to rotor flexibility and stiffness of active magnetic bearings and another operational conditions. Also the results reveals that the significant changes in the chaotic regions in new 8 D.O.F model. Comparing the results with other references revealed good matching. It implies the accuracy of this model in this research. Chaotic vibrations should be avoided as they induce fluctuating stresses that may lead to premature failure of the machinery’s main component. It will be beneficial to the design of AMB systems. Keywords Flexible rotor, Active magnetic bearing, Chaos, Maximum Lyapunov exponent.