The presence of a superstructure is identified to play a key role for the modifications in both superconductivity and structure transitions in the Mg1-xAlxB2 system. This superstructure occurs along the c-axis direction, and can be well interpreted by Al-layer ordering. The optimal composition of the superstructure phase is MgAlB4, a superconductor with Tc ∼12K. Raman spectrum of MgAlB4 gives rise to a sharp peak at around 941cm−1. Brief diagrams illustrating the superconductivity and structural features of Mg1-xAlxB2 (0≤x≤1) materials are presented.