The electrical properties of the surface layer formed on the Nb-doped SrTiO3 single crystals after annealing in air were characterized by the measurements of current-voltage relationship on the junction, (-) Ga-In/bulk/surface layer/Ga-In (+), at low temperatures. Here, Ga-In alloy is an ohmic electrode material. In the forward bias, the tunneling process predominate the current below 130 K, while at high temperatures, the Poole-Frenkel effect determines the current. An anomalous decrease of current under reverse bias with increasing temperature was found. This phenomenon was attributed to the variation of the Fermi energy in the surface layer and the bulk with temperature. (C) 2004 Elsevier B.V. All rights reserved.