The emission mechanisms of rubrene doped molecular organic light-emitting diodes (MOLEDs) is discussed in terms of energy transfer and direct carrier recombination at the dopant. The emission mechanism is investigate by using single layer devices composed of 5,6,11,12- tetraphenylnapthacene (rubrene) as the dopant and N,N'- diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl- 4,4'-diamine (TPD) as the host. Efficient energy transfer form TPD to rubrene is suggested by current-voltage and drift mobility measurements of the doped and undoped TPD films in single layered devices. It is found that electrons can be injected into the hole transporter, TPD. The spatial distribution of the creation of rubrene excitons is studied by change in the thickness sand location of the doped TPD layer in multilayered devices. In rubrene doped TPD, the width of the emission zone extends about 20 nm form the Alq3 interface. In undoped TPD, the diffusion length of TPD exciton is found to be no wider than 5 nm. The penetration depth of the electron injection into undoped TPD is found to be <EQ 5 nm from the Alq3 interface. By rubrene doping, the penetration depth of electron injection seems to be extended beyond 5 nm. The dominant emission mechanism for rubrene-doped TPD is attributed to the electron-hole recombination on rubrene molecules.