The developing processes of microstructure of NdDyFeB green compacts in the different heating stages have been analysed in a thermodynamic principle, and some kinetic phenomena such as particle rearrangement, solution-precipitation, formation of solid skeleton and ploygonization have also been researched. It is shown that the density of magnets varies with the change of microsturcture, leading to the change of magnetic properties. The presence of Nd-rich liquid phase, the elmination of gas absorbed film on the surface of grains of different sizes are helpful for densification of magnets. The grain coarsing, polygnonization of grain shape and pore migration are keys to obtain the final densification.