Polycrystalline Fe‐Mo‐Si‐B alloys with grain sizes of 15–200 nm were synthesized via crystallization of the amorphous alloy. Positron lifetime results show that there are two existing types of interfacial defects in nanocrystalline Fe‐Mo‐Si‐B alloys, namely free‐volume‐sized defects and nanovoids. The free‐volume‐sized defect shows almost no change in size or density during grain growth, and has a mean lifetime smaller than that of its amorphous counterpart as a result of structural relaxation in the process of crystallization. However, abnormal changes in size and density of nanovoids with grain growth were noticed. It is of great interest that the variations of intermediate lifetime &tgr;2and intensity ratioI1/I2with the average grain sizeD¯ are exactly compatible with those of microhardness. A lower density of nanovoids corresponds to a larger microhardness in the case of the present alloy system.