In this work we explore the influence of the exciton recombination zone (RZ) on magnetic-field effect in tris-(8-hydroxyquinolinato) aluminum (Alq3) based organic light-emitting diodes by changing the thickness of Alq3. The magneto-electroluminescence and magneto-conductance (MC) in these devices are investigated at various temperatures and bias voltages. It is found that the sign of MC changes from positive to negative, and then back to positive with the reduction of the thickness of Alq3 at 50 K. The phenomenon observed is ascribed to the change of the exciton density in the exciton RZ. Based on the mechanisms including the hyperfine mixing, the triplet-charge interaction and interfacial dissociation or quenching of excitons, the observed results are explained qualitatively.