Today, with the increase in accuracy and speed of rotating machines, the analysis of bearing vibrations in dynamic rotor systems has become one of the most important needs of the industry. The use of high-speed rotary machines, in addition to the initial problems including the high cost of construction, installation and maintenance, inevitable problems such as reaching the system at critical speeds and consequently the instability of the whole system is possible. Low nominal values in such machines can produce high-amplitude vibrations at high rotational speeds. Therefore, in order to control the vibrations of rotating machines and reduce the resulting effects, journal bearings are added to these systems as auxiliary components. In this thesis, the nonlinear behavior of the rotor system with two types of short and long journal bearings is investigated. The system under study consists of a flexible rotor with a rigid disc mounted on two journal bearings. The governing equations on rotors based on the Timoshenko beam theory are extracted and finally the nonlinear equations of motion of the whole system are solved by Newmark method. To identify the nonlinear behaviors of the system, tools such as time waveform, whirl orbits, Poincaré maps, frequency spectrum diagrams and bifurcation are used. Also, the analysis of parameters such as rotational unbalance and rotational speed of the rotor has been performed. The results indicate the existence of periodic and quasi-periodic behaviors in the system that the change of the mentioned parameters can lead to delay, decrease or increase of non-harmonic regions. Applying the obtained results can be useful in improving the vibration behavior of rotating systems and increase the useful life of parts Keywords: Journal bearing, Flexible rotor, Finite Element Method, Bifurcation, Quasi-Periodic vibration