<p>This paper discusses the rotor vibration and bearing temperature instability of centrifugal compressors caused by oil varnish. Although numerous publications have addressed the adverse effects of oil varnish, related studies specifically exploring how oil varnish leads to rotor vibration and bearing temperature instability are scarce. Accordingly, this paper seeks to explain the underlying principle of how oil varnish influences rotor vibration and bearing temperature in centrifugal compressors. First, rotor vibration and bearing temperature instability due to the varnish effect is identified. We find that the presence of oil varnish results in a rapid increase in bearing temperature and vibration, followed by a gradual decrease, with this pattern repeating. Next, the causes of instability are assumed. Finally, finite element analysis is conducted to verify the assumption.</p>