The effects of Nb on the microstructure and impact toughness of sintered Nd-Fe-B magnets as well as their inter relation have been investigated. The results show that the addition of Nb improves the microstructure, and increases impact toughness of sintered magnets. When Nb content of the sintered magnets without Co is 15 at%，the maximum impact toughness is obtained. But for the sintered magnets with the Co addition, the variation of impact toughness of the magnets is not obvious, and the magnets have lower impact energy than the magnets with the Co addition. The microstructure of sintered magnets has been studied. It was found that many factors affect the impact toughness of sintered magnets, but the main factor is the width and shape of grain boundary of sintered magnets.