Carbon/carbon( C / C ) composites were bonded with a phosphate adhesive prepared from H 3 PO 4 (orthophosphoric acid) and A l( OH ) 3 as the matrix and S i and C u O as the inorganic fillers. The mechanical properties were tested at room temperature after the bonded specimens were treated from RT to 1500°C. The results showed that the addition of C u O could greatly improve the adhesive's mechanical properties due to the formation of C u n S i (copper silicides). The shear strength of bonded specimens could be up to 8.2 MPa without heat treatment and remain above 2 MPa within the whole heat‐treatment temperature range.