Microstructural characterization was performed by analytical transmission electron microscopy (ATEM) to clarify the formation mechanism of tin whiskers. A tin (Sn)-layer about 10 μm thickness was electrodeposited on a FeNi42 plate. The Sn/FeNi42 plate was heated and cooled repeatedly in a temperature range of 233-403 K for 100 cycles under various atmospheres. Cross-sectional TEM specimens of whiskers on the Sn/FeNi42 plate were prepared using a focused ion beam (FIB) micro-sampling technique. Some artifact-effects of the FIB treatments on microstructures in tin alloys were taken into account. Bent-type whiskers grown on the Sn/FeNi42 are of polycrystalline β-Sn. A tin-oxide layer around 10 nm in thickness is formed on the tin surface. Electron diffraction, high-resolution TEM and electron energy loss spectroscopy (EELS) analyses revealed that the tin-oxide layer is predominantly of crystalline SnO 2. The presence of crystalline SnO was also recognized. It was found that the tin-oxide layer tends to be rather thin around stems of the whiskers. It is interpreted that thermal stress in the tin-layer breaks the thin tin-oxide layer at grain boundaries of tin and the whiskers grow from the broken area. The formation mechanism of the whiskers is discussed.