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Radial aerostatic bearings (air bearings) are used to support rotors on a thin air film, which is created by externally pressurized air supplied to the clearance through orifices. The friction loss of the air film is quite low due to low viscosity of the air used. So, it is especially used in high-speed applications such as turbines and gyroscopes. Due to high speed, these systems are very sensitive to the unbalanced forces. Moreover, the clearance between bearing and the rotor is generally around 75 μm and this tight working space increase the complexity of the problem further. The unbalance force is rotating with the rotor; continuously changing direction, as well as its magnitude depends on to the angular velocity of the rotor. In this study, the effect of unbalanced rotor supported by a pair of air bearing is investigated. The bearing-rotor system is modelled as a Jeffcott rotor in two degrees-of-freedom and unbalance is modelled as an eccentric mass from the rotation center of the rotor. In order to calculate non-linear air lubrication film forces, the air flow between the rotor and the bearing surfaces is modelled using Reynold’s equation. A numerical solution algorithm is developed for simultaneous solution of air flow and rotor’s equations of motions. Effect of unbalance to the dynamic behavior of the system is analyzed by simulation run at different rotor speed. Simulations are repeated for different values of the clearance to observe different resonance condition of the rotor with an unbalance.