Effect of high temperature (T1) tempering on sintered NdFeB magnets was investigated. With increased T1 tempering of magnets, both Br and iHc are improved after low temperature T2 tempering at 600 oC, and optimum T1 is 900 oC. As T1 increases further, Br and iHc begin to decrease. Magnets experiencing T1 tempering own higher iHc but lower Br compared with magnets of free T1 tempering. Whether the magnets experience T1 tempering or not, both Br and (BH)m increase gradually while iHc increases greatly when low temperature T2 tempering increases. The optimum T2 is 600 oC. Microstructure analysis shows that there is a large amount of non-uniform phases in the main phase of sintered magnets, and they act as nuclear centers of reversal magnetism and result in iHc decreasing. After T1 tempering, the non-uniform phases disappear in the main phase and the grain boundary becomes clear and smooth, inhabiting the interaction between Nd2Fe14B grains. So the value of iHc for the tempered magnets is improved a lot.