We have demonstrated the fabrication and characterization of single-layered hybrid polymer-quantum dot light-emitting diodes (PQD-LEDs) with the emissive composite film of 2,3-dibutoxy-l,4-poly(phenylene vinylene) (DBPPV) and CdSe/ZnS core/shell quantum dots (QDs). The electrical and optical characteristics are significantly influenced by the thickness of the emissive layer. When the thickness of composite film is thinned to about 103 nm, the maximum luminance of 4100 cd/m2 as well as maximum luminous efficiency of 1.35 cd/A can be achieved at 9.6 V and 7.6 V, respectively. However, the electroluminescence spectra reveal that the emission contribution is dominated by the emission from DBPPV. A maximum ratio of emission intensity of QDs to that of DBPPV is about 38%, which indicates the contribution of QDs is minor. In addition, the post-annealing effect on device performance was also investigated. In contrast to previous reports, no obvious improvement can be observed by post-annealing.