Li-stuffed garnet-type Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZNO) and Li7+yLa2.75Ca0.25Zr1.75-yNb0.25MyO12 (M = Mn, Fe, Co, Ni; y = 0.1, 0.2, referred to as yM-LLCZNO) were synthesized in air by ceramic synthesis at different sintering temperatures. The resulting compositions were characterized for their phase formation and microstructure, and their ionic and electronic conductivities in air and 7% H-2/N-2. Powder X-ray diffraction showed that transitionmetal-doped LLCZNO garnets contain several impurity phases. The transition metal-substituted samples have lower conductivities than that of the parent LLCZNO. The electronic conductivities of selected transition metal-doped LLCZNO samples were found to be about 4 to 5 orders of magnitude lower than their corresponding ionic conductivity, highlighting the challenge to design single-phase mixed electronic and ionic conducting Li-stuffed garnets in both oxidizing and reducing atmospheres. The research area of single-phase mixed conducting Li-garnet is relatively unexplored at the moment, and a theoretical study will help to elucidate the underlying challenge in doping transition metal in Li-based garnets structure and its effect on transport properties. (C) The Author(s) 2018. Published by ECS.