Incorporation of SnO2 into MgO-spinel (M-S) increased mechanical properties significantly. Relationships between the parameters improving mechanical properties and microstructural variables were examined. Basic parameters improving the mechanical properties of M-S-SnO2 composites were identified as follows: (a) when microcracks come across with either Mg2SnO4 particles or pores; crack branching and deviation of interlinked microcracks or crack arresting occurred more effectively than those of spinel particles, (b) fracture type was converted to intergranular fracture with incorporation of spinel into MgO, and transgranular fracture with addition of SnO2 to M-S; additionally with the incorporation of additives, (c) critical defect size, (d) work of fracture values increased, and (e) MgO grain size decreased. R-st thermal shock parameter values of M-S-SnO2 composites were markedly higher than those of M-S materials, associated with low strength loss, high thermal shock damage resistance and thus longer service life of M-S-SnO2 composites for high-temperature industrial applications. (C) 2012 Elsevier Ltd. All rights reserved.