Two kinds of atom distribution in Mg-9wt.%Al melt, undissolved particles and atom aggregation, were investigated by synchrotron small-angle X-ray scattering (SAXS) and molecular dynamics simulation, respectively. The SAXS results indicated that no undissolved particles exist in Mg-9wt.%Al melt after short-time equilibration. The simulation results indicated that atom Mg and Al have a tendency to form bonds in Mg-9wt.%Al melt and the sizes of aggregated Al-centred clusters are within the characteristic scale of medium-range order; the degree of atom aggregation in Mg-9wt.%Al melt increases slightly with increasing temperature.