Introducing carbon nanotubes (CNTs) into polymer or ceramic matrices has been apromising approach to obtain ultra-strong, extra-toughened materials as well asmultifunctional composites. Most of the previous work on CNT composites has focused onstrengthening and toughening of matrix materials at ambient conditions. However, so farthere is a lack of information on the mechanical behavior of these composites at elevatedtemperature. Recently, single-walled CNTs were found to undergo a superplasticdeformation with an appealing 280% elongation at a high temperature (Huanget al 2006 Nature 439 281). This discovery implies the high probability for thepotential usage of CNTs as reinforcing agents in engineering high-temperatureceramics with improved ductility. Here, for the first time, we demonstrate that asmall addition of boron nitride nanotubes (BNNTs) can dramatically enhance thehigh-temperature superplastic deformation (SPD) of engineering ceramics. Morespecifically, 0.5 wt% addition of BNNTs leads to an inspiring brittle-to-ductile transition inAl2O3 ceramics even at amoderate temperature (1300 °C). For Si3N4 ceramics, 0.5 wt% addition of BNNTs could also decrease the true stress by 75% under thesame deformation conditions. In contrast, addition of micro-sized or nano-sized BNpowders has no or a negative effect on the superplasticity of these ceramics. The underlyingSPD-enhancement mechanism is discussed in terms of the inhibition of static and dynamicgrain growth of the matrix and the energy-absorption mechanism of BNNTs. Theunraveled capability of BNNTs to enhance the SPD behavior will make BNNTs promisingcomponents in cost-effective complex ceramics with good comprehensive mechanicalproperties.