The efficiency of organic light-emitting devices (OLEDs) is closely related to the position and width of recombination zone (RCZ) in the emission layer. Based on the drift diffusion theory of carrier motion in semiconductors, we developed a numerical model for the position and width of the RCZ in bipolar single layer OLEDs. The calculation results show that for a given operation voltage, the position and width of the RCZ are determined by the mobility difference of electrons and holes, and the energy barrier at the two contacts. When the anode and cathode contact are both ohmic, then RCZ will be near the electrode, from which the low-mobility carriers are injected, and the smaller the mobility difference, the wider the RCZ, and the width of RCZ will be maximal when the mobility of holes and electrons are equal. When the anode contact is Schottky, while the cathode contact is ohmic, then the position and width of RCZ will be determined by both the mobility difference and hole injection energy barrier. When μ p<μ n, the RCZ will be at the anode side. When μ p>μ n, then RCZ will move away from the anode and become wider, with the increase of the hole injection barrier. For a given hole injection barrier and mobility of holes and electrons, the position and width of RCZ change with the applied voltage.