ZrO 2 /multi-wall carbon nanotube (MWCNT) composites were consolidated using a spark plasma sintering technique. Micro-and nano-scale mechanical properties of sintered composites with various MWCNT contents were investigated using X-ray diffraction and instrumented indentation techniques. Transitional behavior in the hardness and elastic modulus of the composites was observed and attributed to the contrasting effects of the MWCNT addition; i.e., beneficial effects such as inhibition of crystal growth and phase transition and detrimental effects due to the agglomerated MWCNTs. ©2014 The Ceramic Society of Japan. All rights reserved.