The behavior of hydrogen absorption and release in hydrogen decrepitation (HD) process of Nd Fe B alloys were investigated. The results reveal that the reactivity and the amount of hydrogen absorption in HD process are related to the surface activity of the alloy so that the fresh and active surface has a higher efficiency. The presence of Nd rich phase at the grain boundary is an essential factor of the HD activity of the alloy at room temperature. On degassing, hydrogen is released from the HD powder continuously with increasing temperature. And the residual hydrogen is as low as 0 0015% at 1073 K, which shows that the hydrogen is almost exhaused. It is feasible to remove the hydrogen from the HD powder by heating treatment at the temperature of 523～723 K for 1 h prior to the magnetic field forming in order to decrease the harmful effect of hydrogen on the easy axis alignment of HD magnet.
