Influence of mixing Co on the structural stability of ultra-thin NiPt silicide film is investigated in this paper. Experiments of variation of the total thickness and Ni : Co : Pt ratio of the Ni, Co and Pt mixtures are carefully designed and carried out. Compared with the 6-nm NiPt sample silicide film, films of Ni, Co and Pt mixtures with similar total metal thickness demonstrate ~150 °C higher agglomeration temperature. The agglomeration temperature and phase transformation characteristics are found to be relatively stable, regardless of the variation of the total thickness and Ni : Co : Pt ratio in the Ni, Co and Pt mixtures.