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.