We studied the effect of thin film thickness on the performance of double-layer heterojunction organic light-emitting diodes (OLEDs) based on 4,4'-bis biphenyl (NPB) as hole transporting layer and tris-(8-hydroxylquinoline)-aluminum (Alq₃) as emitting layer.Various devices with different NPB and Alq₃ thickness were fabricated and their electroluminescent characteristics were investiagted.The results show that the device current dosen’t change with Alq₃ or NPB film thickness in the same trend.The current mechanism of devices with different organic film thicknesses conforms with the theory of trapped charge limited current(TCLC).Current varies from ohmic conduction region,TCL region to trapped charge limited-space charge limited (TCLC-SCLC) transition region with the increase of applied voltage.High device performance could be obtained when thin film thickness is NPB(20 nm)/Alq₃(50 nm).The luminance efficiency-voltage curves of devices reached to the maximum under low-voltage,and gradually decreased with rising voltage.