Iron is an element to harm the corrosion resistance of magnesium. Thus, it is always necessary to keep in mind the iron behavior on the properties of magnesium. However, there are a few examples reported about an iron state in pure magnesium. We conducted this study to determine the solubility of iron in pure magnesium and to clarify the cast structure of Mg-Fe alloy. Iron was added by immersing a pure iron sheet in the molten pure magnesium at temperatures of 963, 998, and 1073K. After the melt was held at the selected temperature for 86.4ks at maximum, a 5ml specimen was collected from the melt and subjected to chemical analysis. The experimental values were plotted on graph, LogN versus 1/T. We formulated the regression line into the numerical expression represented by following equation: LogN = 3.931 - 7981/T where N is the solubility of iron in the liquid pure magnesium (molar fraction), and T is the absolute temperature. This equation is an experimental formula expressing the solubility of iron in pure magnesium. According to this equation, iron concentration at eutectic point was estimated to be less than 0.018mass% (eutectic composition in common phase diagram). However, we confirmed primary iron formed well-defined dendrites in the furnace cooled Mg-0.011mass%Fe alloy.