Unsteady static pressure signals due to flow instability in two types of centrifugal compressors were analyzed by employing the phase portrait reconstruction method. The sampled data corresponded to several streamwise locations along the shroud wall over a wide range of operation from design to near surge. Singular value decomposition analysis yielded successfully the discernable features of flow instability, i.e., stall and surge, which were observed with a decrease of mass flow rate. The effects of the signal-to-noise ratio was found to be the most troublesome in predicting the onset of flow instability upon pursuing the attractor behavior of the portraits. Under the latter difficult circumstance, the correlation integrals were also conveniently calculated to help to check the onset. It was clearly indicated that the behavior near rotating stall was not always recognized by the phase portrait in three-dimensional space, while the corresponding correlation integral obviously decreased close to stall. Monitoring of unsteady signals based on the phase portraits and the correlation integrals, therefore, led to a good judgement of a nonlinear fluid dynamic system response and to prevent compressors from a disastrous damage due to flow instability.