Sm-Fe-N alloy with Zn at the grain boundaries has high magnetic coercivity. Most studies on this type of alloy have focused only on Zn in rhombohedral crystal Sm-Fe-N powders. The behavior of Zn in hexagonal polycrystalline Sm-Fe-N powders has not been characterized by researchers, much less the dynamic diffusion of Zn into polycrystalline Sm-Fe-N during heating. Here, we directly visualized Zn diffusion into hexagonal polycrystalline Sm-Zr-Fe-Co-N using a transmission electron microscope equipped with a heating holder. Zn on the surface of Sm-Zr-Fe-Co-N disappeared with subsequent changes in the grain boundaries during heating, indicating that Zn preferentially diffused into the grain boundary to form the Fe3Zn10 phase. This technique can be useful to elucidate synthesis reactions under specific environments, leading to optimization of the material synthesis technique.