Single crystals of magnesium diboride with maximum dimension of 200×200×60μm3 have been successfully grown from copper flux in iron capsules. MgB2 crystallized separately in a system with the composition Mg40Cu40B20, and large single crystals were grown. Crystals grow together in the system with the composition Mg35Cu40B25, and MgB4 is found in this system due to excess B content. MgB2 crystallizes in very small particles in a system with the composition Mg50Cu 40B10. MgB2 single crystals show strongly polarized light properties, and thus are easily discriminated from other kinds of accompanying crystals and from their surroundings. Magneto-optical microscope gives direct evidence of superconducting properties of MgB 2 crystals. Further measurements of ac-susceptibility show that crystals begin the superconducting transition at 39.1K with a transition width of 1.3K. © 2003 Elsevier B.V. All rights reserved.