This study presents the measurement techniques on the periodic fluctuating flow such as the discharge flow of a centrifugal impeller in an unstable operating region. During rotating stall, the flow at the exit of a centrifugal compressor impeller fluctuates periodically with a lower frequency than that of the blade passing. To observe the blade-to-blade flow characteristics during the rotating stall, the phases of all the sampled data sets should be adjusted to those of the reference signals with two processes, in these processes, DPLEAT (Double Phase-Locked Ensemble Averaging Technique) can be used. From these measurements and data processing techniques, the characteristics illustrated a blade-to-blade flow with high frequency, but also a periodic rotating stall flow with a low frequency at the centrifugal impeller exit which was clearly observed.