Abstract: The discharge characteristic and plasma parameters of electrical explosion of aluminum thin wire were studied by framing camera and spectrograph. The experiments were carried out under several different discharge voltages with aluminum wires of 40 m and 100 m in diameter. The experiments show that there are two types of secondary breakdown processes in the metal vapor: inner breakdown and surface breakdown. The inner breakdown, which generates a plasma with better spatial uniformity and symmetry and consequently leads to more steady and repeatable discharge, takes place more easily in thinner wire under the same conditions. The electron temperature and density of the plasma have the order of magnitude of 104 K and 1018 cm-3, respectively.