Dislocation cells, geometrically necessary boundaries (GNB) and twins in drawn single crystal copper wires produced by Ohno continuous casting (OCC) method have been analyzed by TEM. The results show that when the strains are lower than 0.94, the microstructures of drawn single crystal can be characterized as dislocation cells. When the strains are more than 0.94, there are two kinds of microstructures. One is that dislocation cells are elongated in longitudinal section and equiaxed in cross section. The size of dislocation cells decreases as the strain increases until they reach a critical size of about 0.28 μm at a strain of 1.39. The other is geometrically necessary boundaries. At low strains, two kinds of geometrically necessary boundaries are along {111}, and at high strains, geometrically necessary boundaries are parallel to drawn direction. In addition, it is found that in dawn single crystal copper wires, there are two types of twins in the single crystal copper wires. One is formed in the preparing process of single crystal copper wires by Ohno continuous casting method, and the other is formed during cold drawing deformation.