Grain growth of the Nb3Sn at the Nb/solid Cu-Sn and Nb/liquid Cu-Sn interface has been investigated. Experiments have revealed that the grain growth of the Nb3Sn at the solid/solid interface still conforms to the ordinary law of the grain growth in metals, though the Nb3Sn grain size (～0.1 μm) is much smaller than that of the metals. The Nb3Sn layer growth at the solid/liquid interface is divided into two layers: (1) near the Nb, the grains are fine and closely packed, (2) near the liquid Cu-Sn, the grains are coarse and dispersive. The coarse grains, are thought to be formed by a solution/deposition process. Most of the Nb3Sn crystals have the appearance of rhombic dodecahedron and orthogonal parallelepiped. It is evident that the interfacial energy of the 110 and 100 planes of the Nb3Sn crystal is lower than the others.