The precipitation behavior of second phases in Hastelloy C-2000 alloy matrix was studied after isothermal oxidation at 800 oC for 100 h in air. Mo-rich phases were precipitated in the alloy matrix after oxidation. It is determined that Mo-rich phases with diamond cubic structure are Mo3Ni3C type carbides. Morphology characteristics of Mo-rich phases with both no-continuous irregular strip and continuous were presented along grain boundaries and in matrix, respectively. However, some variation of morphologies will take place at both grain and grain boundaries after deep etching, i.e., a large number of white flocculent structure were distributed along grain boundaries, and corrosion pits with a diamond structure appear in intragranular. Areas of Ni and Cr-rich are more easily corroded during deep etching due to a more negative electrode potential in areas.