It is well recognized that 10% Hydronalium alloy reveals the highest mechanical properties among the sand casting aluminium alloys after appropriate solution heat treatment. But sound castings and higher mechanical properties are expected only by best foundry practices and adquate heat treatments.In the first place, in order to clarify the effect of maximum heating temperature and casting temperature on the final mechanical properties, the molten metal was cast at various tempreatures in heating cycle to maximum tempereature as well as in cooling. The mechanical properties scarcely vary owing to the maximum melting temperature.To obtain the best properties of 10 per cent Hydronalium, temperature of solution heat treatment becomes essential, which bring β phase (Al3Mg2 or Al8Mg5), Mg2Si and Al3Fe compounds into single α solid solution. So it becomes necessary for us to definite the crystal structure of β phase itself to investigate the process of heat treatment.After surveying X-ray diffraction patterns of polycrystalline β phase, the authors confirmed that the β phase was f.c.c. lattice with very large lattice parameter, and it was also revealed that the atomic arrangement of the phase fluctuates by trace impurities.As homogenization during solution heat treatment is considered to be affected by the temperature and time of holding, dilatometric test, X-ray analysis and optical microscopic observation have been carried out in order to confirm above effects (during solution heat treatments).In the course of solution treatment, contraction of the specimen was observed, which seems to explain the disintegration of β phase into α solid solution. The higher the temperature, the more rapid the shrinkage proceed, however, volume change is not complated until after 30 hours. Moreover, we have performed supplemental experiments to investigate the effect of magnesium content, time of holding and cooling media (air, boiling water) on the mechanical properties, and on corrosion resistance.The addition of a large quantity of magnesium improves tensile strength though elongation decreases greatly, when magnesium content exceed 12 per cent, the mechanical properties found to be rather unstable. Quenching in boiling water developed better results than air cooling.