Using molecular dynamics method with many-body potentials, we have studied the thermal stability and melting mechanism of platinum nanocrystal of truncated octahedron enclosed by 111 and 100 surfaces, and investigated its microstructure and shape by statistical radius and Lindemann index. The results show that the nanocrystal first shows shape changes at 1500 K, then transforms to a spherical one at 1700 K. The surface first shows the premelting behavior at 1500 K, and the melt completely into liquid state. The melting process starts from the surface into the interior at this temperature, resulting in the complete melting of the nanocrystal at 1730 K. The surface premelting is advantageous to shape transformation of octahedron-truncated nanocrystal.