Under the pressure of 77 kb, double layer structure always appears in the polycrystal diamond grown with wireform catalyst. The distribution of catalyst metal nickel in specimen has been in vestigated.The appearence of partial precipitate peak of catalyst metal has been explained by means of the forced diffusion equation of phase transformation growth. The cause of the presence of metal partial precipitated zone, the relationship between the double layer structure and the P-T growth region have also been discussed.