The effects of psst-sintering heat treatments on the structure and properties in sintered Nd15Fe77B8 magnets have been investigated. In the as-sintered sample, the crystal structure of the Nd-rich phase was f.c.c., while the Nd-rich phase transformed into a superlattice after heat treatment at 600 degrees C for 1 h. However, the structures of Nd2Fe14B and B-rich (Nd1+epsilonFe4B4) phases did not change upon heat treatment. It is suggested that the formation of the superlattice enhances the coercivity of the magnets.