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We investigate the high-temperature compressive deformation behavior of a novel, fully dense and structurally uniform, 20 vol% multiwalled carbon nanotube (MWCNT)–α-Al2O3 matrix hybrid, which has a strong room-temperature interfacial shear resistance (ISR) and a unique MWCNT-concentrated grain-boundary (GB) structure. We realized a perfect plastic deformation at 1400°C and a rather high initial strain rate of 10−4 s−1 by a low ~30 MPa flow stress, which is contrary to the strain hardening response of fine-grain monolithic Al2O3. This unique performance in CNT–ceramic system in compression is explained as follows: the concentrated network of individual MWCNTs perfectly withstands the high-temperature and shear/compressive forces, and strongly preserves the nanostructure of Al2O3 matrix by preventing the dynamic grain growth, even during a large ~44% deformation. Furthermore, the presence of large amount of radially soft/elastic, highly energy-absorbing MWCNTs in the GB and specially multiple junction areas, and a potentially weak 1400°C-ISR, could greatly facilitate the GB sliding process (despite the hybrid's strong room-temperature ISR), as evidenced by the formation of some submicrometer-scale MWCNT aggregates in GB area, the equiaxed grains and dislocation-free nanostructure of the deformed hybrid. The results presented here could be attractive for the ceramic forming industry and could be regarded as a reference for oxide systems in which, the GB areas are occupied with soft/elastic, highly energy-absorbing nanostructures.