A novel soluble phenothiazine-based derivative, poly(3, 7-divinylene-N-octyl-phenothiazine 2, 3-diethylquinoxaline-5,8- diyl) (PQDN), is synthesized for application in red light-emitting diodes. PQDN was characterized by the measurements of cyclic voltammetry (CV), absorption and photoluminescence (PL) spectra. The band gap (Eg) was determined to be 1.89 eV. The smaller band gap suggested that the polymer is a good candidate for red electroluminescent materials. Polymer light-emitting diodes (PLEDs) with double-layer configuration of indium tin oxide (ITO) / (polyethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) / PQDN: poly(N-vinylcarbazole) (PVK) (100:0, 80:20, 60:40, and 20:80 wt.%) / Mg:Ag were fabricated. The electroluminescent (EL) property was studied in details. In order to improve the EL performance further, the devices were thermally annealed at 100oC for one hour under the protection of nitrogen. Luminance is increased obviously. EL peak wavelength and the Commision International de L'Eclairage (CIE) coordinates are unaltered and located in red region with the increase of the operating voltages,.