In this letter, the clutter rank estimation formulas for endfire array airborne radar (EFAAR) in two typical scenarios are derived. In comparison to the airborne radar with a traditional broadside array, the clutter rank of the EFAAR in the forward-looking scenario still maintains a reasonably accurate estimation criterion. The effectiveness of the proposed clutter rank formulas is validated, and the impacts of certain parameters on the clutter rank are analyzed through simulations. Simulation results indicate a direct proportionality between clutter rank and the number of array elements, coherent processing pulses, and clutter ridge slope.