Organic light-emitting device (OLED) based on Alq_3 emitting layer, ITO transparent anode and NPB hole transport layer were fabricated. The structure was ITO/NPB/Alq_3/LiF/Al. Optimized thickness 65 nm and deposit speed 0.3 nm/s were confirmed through the comparison of brightness and luminescent efficiency with different thickness and deposit speed of Alq_3. The threshold voltage 3.01 V,brightness 12 800 cd/m~2,efficiency 4.421 cd/A were achieved. In addition, influence factors were (analy)zed.