We have studied the surface structures of Al/Si(100) system by observing the I-V curves and intensity line profiles of LEED. For the 2×3 phase a streaky LEED pattern was observed, and it is interpreted that Al dimer rows are not arranged perfectly in every other Si dimer. The resemblance of I-V curves of the integral order spots between the Al-4×1 and Al-7×1 phases indicates the correlation of the atomic geometry of the Si substrate between the Al-4×1 and Al-7×1 structures.