The effects of plasma treatment on the ITO/glass substrate before deposition of organic materials were investigated in the fabrication of green light emitting organic devices with $Alq_3-C545T$ fluorescent system. In our experiments, the optimum plasma treatment was obtained at the power and time of 150W and 2 minutes under the $Ar(50%)/O_2$ ambient of 1 mTorr. The green OLED with plasma treatment at 150W for 2 minutes showed the luminance and efficiency of $4700;cd/m^2$ and 8 lm/W at 10V, respectively. On the contrary, the same structured device without plasma treatment showed much lower performance with the luminance of $2600;cd/m^2$ and the efficiency of 3.6 lm/W at 10 V.