Abstract: By means of TEM, TEDAX and AES the chemical composition and precipitates on particle surface and at the primary particle/particle boundaries of HIPed billet have been investigated. Experimental results show that the particle surface of powders produced by Ar atomisation is enriched with S、C、Ti、O and TiC particles also precipitate on the surface. After hot isostatic pressing, the carbides distributed around the primary-particle boundaries are niobium ri-ched (Nb, Ti, Mo) C phase, S and O concentration drastically decreases at the primary-particle boundaries, and a small amount of P segregate to the primary-particle boundaries during hot isostatic pressing.