In this study the effect of nano meter size ZrO2 particles on the microstructure, densification and hydration resistance of magnesite –dolomite refractories was investigated. 0, 2, 4, 6 and 8 wt. % ZrO2 particles that were added to magnesite –dolomite refractories containing 35 wt. % CaO. The Hydration resistance was measured by change in the weight of specimens after 72 h at 25℃ and 95% relative humidity. The results showed with addition of nano meter size ZrO2 particles, the lattice constant of CaO increased, and the bulk density and hydration resistance of the specimens increased while apparent porosity decreased. With the addition of small amount ZrO2 the formation of CaZrO3 phase facilitated the sintering and the densification process. The mechanism of the nano meter size ZrO2 particles promoting densification and hydration resistance is decreasing the amount of free CaO in the specimens.