A comparative study of in situ postgrowth annealing of organic layers before metal cathode was conducted on tris-(8-hydroxyqunoline) aluminum (Alq3)-based organic light-emitting devices (OLEDs). The devices were fabricated in the same run with a standard device without annealing for comparison, with an identical structure of indium tin oxide (ITO)/copper phthalocyanine (CuPc) (10 nm)/N,N′-di(naphthalene-l-yl)-N,N′-diphenyl-benzidine (NPB) (90 nm)/Alq3 (90 nm)/Mg:Ag (200 nm)/Ag (20 nm). The annealing temperature used was 60, 80, and 100 °C, respectively. It was found that, in situ postgrowth annealing improves the device performance, and annealing near the glass transition temperature of NPB (99.7 °C), improves device performance drastically. Power efficiency and current efficiency increase significantly with the annealing temperature, except the current efficiency for device annealed at 100 °C is slightly lower than that of the standard device. The voltage and current density for 100 cd/m2 luminance are 5.6 V and 4.4 mA/cm2, respectively, for the device annealed at 100 °C, in comparison to 9.2 V and 4.3 mA/cm2, respectively, for the standard device, the power efficiency is much improved by more than 40%. The in situ postgrowth annealed organic layers were characterized by photoluminescence and Raman spectroscopy.