The irreversible flux loss (FL) of sintered Nd-Fe-B magnets has been investigated after aging for about 920 d. Exposed at room temperature (RT) and 353K, the values of FL are about 1.4％ and 13%, respectively. However, with a pre-aging the FL are about 6‰ and 6.9% for the magnets exposed at RT and 353K, respectively. Therefore, the stability of flux has been improved obviously by the pre-aging. A phenomenological formula, based on the Boltzmann statistics, has been used to discuss the effect of microstructure on FL. It is found that the experimental results can be well explained by the formula.