Abstract: The densification process and equilibrium relations of ferrites are discussed.It is pointed out that the relative density of ferrites after sintering should be reached the value of 95%~96% theoretic density in order for HIP without encapsulating layer.It could be fully densified at the temperature 150~160℃ below sintering temperature under the pressure of 100 MPa.Oxidation,cracking and the deterioration of permeability can be found due to residual oxygen in argon and can be prevented by controlling partial pressure of oxygen.The permeability can also be recovered by annealing in balanced atmosphere.