Open-cellular AZ91 magnesium foams with various geometric characteristics, that is, a round-shape cell structure, a diamond-shape cell structure and a square-shape cell structure are produced by a casting method and mechanical properties of these magnesium foams are investigated by compressive tests. The flow stress changed significantly with straining in the foam with a square-shape cell structure. In the foams with a round-shape cell structure and a diamond-shape cell structure, however, the flow stress tended to be stable in the plateau region. These results indicate that mechanical properties of foams are affected significantly by the cell structure.