A chemical synthesis was performed of an octapeptide with a phosphonic ester linkage, which corresponds to a variation of the partial sequence of a gene product of erb B-2. The phosphonic ester linkage was successfully prepared through a coupling reaction of magnesium salt of the hydroxypeptide with phosphonochloridate, without damage of the peptide bond. The free aminophosphonic peptide as the final product gave a single peak on the ODS column, and a reasonable ratio of the component amino acids. The reaction series exploited here might be useful as a general procedure for preparation of the phosphonic peptide.