Organic light-emitting devices (OLEDs), made of polymer bi- layer thin films, on the thin flexible plastic substrates have been fabricated and studied. The ITO and Ca/Al electrodes have been used as anode and cathode, respectively, in this structure. Both polymers -- the hole- transporting and electron-transporting/light emissive co- polymers -- used in this study demonstrated a very good thermal and photo-oxidative stabilities. The hole transporting co-polymer (amine-fluorene) thin film is amorphous, while the light emissive/electron-transporting co-polymer (benzothiadiazole-fluorene) thin film is partially crystalline. The photoluminescence (PL) and electroluminescence (EL) emission peaks are both located around 550 nm. The plastic substrate coated with ITO used in this study is less than 0.2 mm thick and highly flexible. It has a sheet resistance in the range of about 10 to 20 (Omega) /D'Alembertian, and the transmittance higher than 80% in the photon energy ranging from 470 to 750 nm; the maximum transmittance is approximately 84.5%. The root-mean-square (RMS) of the ITO surface roughness is approximately 1.70 nm. The oxygen and water vapor transmission rate through the flexible plastic substrate was about 14.2 X 10-6 cc/cm2(DOT)day(DOT)atm and 1.14 X 10-4 gm/cm2-day-atm, respectively. For the OLEDs fabricated on the thin flexible plastic substrates, the typical turn-on voltage is less than 6 V, and maximum brightness is higher than 13,000 cd/m2. The maximum emission efficiency, luminous efficiency, and external quantum efficiency for our OLEDs are 66.2 cd/A, 18.3 lm/W, and 16.2%, respectively.