Electrical transport properties were investigated in a single crystalline SrTiO3 sample. The electronic conductivity was determined to be 4.0 x 10(-3) P-O2(1/4) + 3.0 x 10(-6) P-O2(-1/4) at 1173 K by a four terminal method. The sample was placed under an oxygen potential gradient, and the electromotive force and the current-voltage behaviour were investigated. Oxide ion conductivity was estimated to be 2.5 x 10(-5) S cm(-1) comparing the emf data with the theoretical values. The average conductivity was explained by a calculation based on local thermodynamic equilibrium and local charge neutrality. Under high applied voltages, a nonlinear current-voltage relationship was observed. Attempts were made to explain the nonlinearity considering the redistribution of oxide ion vacancy in the bulk and oxygen blocking effect at the electrode. The oxygen potential at the polarized electrode was estimated and compared with the oxide ion current.