Abstract: In order to identify suitable pack compositions for co-depositing Cr, Si and Y to form diffusion coatings on Nb-base in situ composites by the pack cementation process, thermochemical calculation was taken to analysis the vapour pressure of halide species generated in a series of pack powder mixtures activated by NH₄Cl or CrCl₃·6H₂O at high temperatures. The Cr-Si-Y co-depositing coating layers on the Nb-base in situ composites were prepared according to the analysis. The microstructure, phase constituents and compositional distribution in the coatings were studied. The results show that by means of carefully controlling the composition of the packs activated by NH₄Cl, co-deposition of Si-Cr-Y on Nb-Si in situ composites can be achieved at the appropriate temperature. It is demonstrated that the co-depositing coating can be formed at 1350℃ with the pack power mixtures composed of 12Cr-6Si-0.75Y₂O₃-5NH₄Cl-76.25Al₂O₃(mass fraction). The coatings obtained have a multiple layer structure, consisting of an outer layer, an inner layer and a diffusion zone at the boundary between the coating and the substrate. It is suggested that the coating is formed via a sequential deposition mechanism through inward diffusion of Cr, Y and Si. The coatings are mainly composed of Cr₂(Nb,Ti), (Nb,Ti)₅Si₃ and HfSi₂, and the microstructure of the coatings can be refined by addition of Y₂O₃ in the pack mixtures.