Abstract: A simplified formula for estimating the instability threshold of aerostatic journal bearing system supported elastically by means of O-rings are derived. Also, experimental investigation has been carried out to estimate the instability threshold. The obtained data of the critical rotation frequency at the instability threshold are given in Fig. 3. The results calculated by the formula are compared with the experimental data graphically.The effects of supply pressures from 196.1kPa to 686.5kPa and of radial clearances from 0.02mm to 0.06mm for three kinds of rubber are investigated and therefrom the following conclusions are drawn:（1） The instability threshold falls with increasing he rubber hardness for the bearing with large clearance at low supply pressures. It is shown that the soft rubber is superior to the hard one in the above-mentioned case.（2） For each rubber tested there exists an optimum bearing clearance. Its threshold of whirl instability is the highest. Moreover, It is much higher than that of the rigidly supported bearing.