Abstract: Reduced-rank STAP has been a key technology in air-borne radar to cancel the clutter. Several methods have been presented through these years. In this paper, those methods are classified into two basic forms-one is based on the generalized sidelobe canceller(GSC), the other is based on the direct form. The differences in the performances and potential of these two catalogs of methods are compared through theoretical analysis. Simulation results in ideal situations and with limited sample are presented.