The emission color of organic electroluminescent devices can be tuned by doping the active layer with certain dyes or other fluorescent molecules. We demonstrate the realization of green PLED by doping a blue light emitting polymer poly(N-vinyl-carbazole) (PVK) with a novel green light emitting molecule tetra-methylester of perylene-3,4,9,10-tetracarboxylicacid (TMEP). At TMEP 0.01 (mass ratio) doping concentration, excited by 295.5 nm, photoluminescence clearly show two emission peaks belonging to PVK and TMEP. For TMEP concentrations between 0.05～0.10 (mass ratio) the photoluminescence (PL) and the electroluminescence (EL) spectrum of the TMEP/PVK blend are almost dominated by TMEP emission because of an excitation energy transfering from PVK to TMEP. The reason for the efficient energy transfering can be explained by the efficient F??rster energy transfering from PVK to TMEP. PL spectra redshift with the increasing concentration of TMEP is due to the excimer formation. The difference between PL spectra of TMEP solution and PL spectra of TMEP solid film confirms the formation of excimer.