Polymer doped small-molecule light-emitting diodes have been fabricated using a novel PTPD (poly-TPD) as the hole transport material and the highly fluorescent rubrene as the dopant. Electroluminescent characteristics are studied at different doped concentrations and different film thicknesses. The results indicate the presence of doping trap effect. It is assumed that the traps are limited at discrete levels. Based upon the double-carrier injection theory, an analytical model of J-V characteristics of doped device is deduced by solving the Poisson equation. Calculated data of this model are consistent with our experimental results.