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In order to assess service properties of 2124-T851 aluminum alloy thick plate in short transverse orientation (S direction) under ultra-high loading, the extremely low cycle fatigue (ELCF) behavior was investigated by controlling nominal axial strain in fatigue experiments of hourglass-type specimens. The results indicate that the 2124-T851 thick plate in S direction exhibits stable cyclic softening in cyclic stress-strain hysteresis loop, and the softening rate can be expressed as a typical?power function of the nominal strain amplitude (Δεn/2). Manson-Coffin formula expressed in Δεn/2 has a good prediction of the ELCF life as compared with the experimental results. The fracture morphology shows that the crack propagation direction tends to follow longitudinal-transverse direction (T direction) in both the monotonic tensile samples and the ELCF fracture samples. With the decrease of Δεn/2, the fracture mode gradually changes from shear mode into normal mode. ELCF microscopic morphology generally?exhibits hybrid fracture?characteristics?of?dimples and extrusion flat caused by two crack surface peened each other. The fatigue striations were found only when Δεn/2 decreased to 0.4%.