We demonstrate here balancing between hole and electron injection in bilayer organic tris(8-hydroxyquinoline)aluminum (Alq3) based electroluminescent (EL) devices by measuring current-voltage (I-V) characteristics of single-carrier devices with different thickness and by comparing the device performance with various chemically modified ITO. We measure I-V characteristics due to holes in hole-only bilayer devices with various modified ITO anodes and those due to electrons in electron-only single-layer devices with C6H5COOLi(2 nm)/Al cathode. Here, hole-only devices are those, in which only holes are carriers and the current due to electrons is negligible. We found that the I-V characteristics observed for the hole-only bilayer devices with UV-ozone and ClC6H4COCl treated ITO were comparable to the I-V characteristics for the electron-only single-layer device of ITO(as-cleaned)/Alq3(150 nm)/C6H5COOLi(2 nm)/Al. In other words, hole and electron injections are well balanced when we use UV-ozone and ClC6H4COCl treated ITO anodes. EL characteristics have shown that luminance decreased clearly for bilayer organic EL devices with an Al cathode where electron injection was not enhanced significantly. The decrease in EL efficiency can be attributed to unbalanced hole and electron injection. The unbalanced injection results in excess of one carrier type that does not contribute to light emission, and results in deactivation of 1Alq3 at high currents due to quenching 1Alq3 by large excess of Alq3 +. in the emission zone.