Using in situ transmission electron microscopy, we report the observation of the melting behavior of one-dimensional nanostructures of Sn with different length/width aspect ratios. The melting of small aspect-ratio nanowires (nanorods) results in the expansion of liquid Sn along both axial and radial directions with the tendency to form an isometric or spherical particle, thereby minimizing the total surface area. For nanowires with the length/width aspect ratio of ∼10.5, perturbation along the liquid stream causes an unstable necking phenomenon and the whole wire tends to shrink into a spherical particle. In contrast, Rayleigh instability sets in for the melting of the nanowires with the length/width aspect ratio as large as ∼21, which gives rise to necking and fragmentation of the wire into particles. The amorphous native surface oxide (SnO
) layer serves as a confinement tube and plays an important role in the melting induced morphological evolution of Sn nanowires. A thin SnO
layer is flexible with the ability to shrink or expand upon the flow of molten Sn. The increased rigidity for a thick SnO
surface layer kinetically suppresses bulging and necking formation in molten Sn nanowires.