Ternary compounds Mg(Mg1-xAlx)Si (0.3 < x < 0.8) have been prepared under high pressure and high temperature conditions of 5 GPa at 800-1100 degrees C. The single crystal study revealed that the compound (x = 0.45) is isomorphous with the anticotunnite, or the TiNiSi structure, and crystallizes with space group Pnma, with lattice parameters a = 6.9242(2), b = 4.1380(1), c = 7.9618(2) angstrom, and Z = 4. The compound with x > 0.5 shows superconductivity with a transition temperature (T-c) similar to 6 K. The compound is a peritectic solid solution associated with other phases such as Mg9Si5, Al, and Si, depending on cooling protocols in the preparation. The band structure calculation on the composition MgAlSi suggests that the Al and Mg orbitals mainly contribute to the density of states near the Fermi level, and the substitution of Mg with Al favors the superconductivity.