Abstract This paper starts with the discussion of the principle of reduced-rank(RR) Space-Time Adaptive Processing(STAP). It is followed by a dedication of the performance upper bound of all eigen-based RR methods provided by Cross Spectral Method(CSM) under the condition of a given processor rank and identical secondary sample size. A performance com-parison between two RR STAP processors with prefixed structure and CSM is performed by the means of simulations. It is shown that the performance of time pre-filtering followed by jointly localized STAP structure (i.e. 3DT-SAP) is very close to the upper bound and thereby it is an effective RR approach.