In situ environmental transmission electron microscopy (ETEM) observation was used to reveal Sn inside Sn-filled carbon nanofibers (CNFs) during heating. Before and during heating, the Sn was covered completely by CNFs. The volume of Sn present in the Sn-filled CNFs gradually decreased during heating, with new Sn particles appearing at the top of the Sn-filled CNFs. It is thought that Sn diffused out of the Sn-filled CNFs and into the carbon wall of the metal-filled CNFs (MFCNFs). The specific surface area at the top of Sn-filled CNFs is higher specific surface than that at other parts and the temperature at the top of the MFCNF is lower than that at other parts; therefore, diffused Sn particles were formed at only the top of the Sn-filled CNF. Fabrication of Sn metal particles was tried by heating the Sn-filled CNFs, and it was clear that the metal particle size could be controlled by controlling the heating time.