There have been lots of studies to control the poor hydration resistance of dolomite refractories, one of the most effective solutions has been the addition of Magnesia to doloma. Using a co-clinker of Magnesia-doloma as a starting material would provide more homogeneity in the properties of the product and has been published recently. On the other hand, addition of iron oxide to doloma has been found to increase the hydration resistance. In this paper, the effect of iron oxide addition on hydration phase analysis and microstructure of two different Magnesia-doloma samples, one with CaO content of 25 wt% and the other one with that of 35 wt% has been investigated. Ten samples were prepared by pressing followed by firing at 1750 oC for 3hrs. Results showed that the hydration resistance of the samples improved by decreasing the CaO content, because CaO is much more prone to hydration comparing to MgO. Besides, iron oxide addition lead to the formation of iron-containing phases which increased the hydration resistance of the samples both by capsulating the CaO and MgO grains and by promoting the liquid phase sintering.