Here we report a red organic electroluminescent device (OLED) with an emission wavelength at 705 nm in which PCDTBT is used as an emitting layer with the structure ITO/PEDOT:PSS/PCDTBT/BCP/LiF/Al. The device shows good performances such as an onset voltage of 2 V, a maximum brightness of 29000 cd/m2 at 9 V and a maximum current efficiency of 3.5 cd/A. The effects of annealing on the luminescent property of the device are studied at different temperatures. The experimental results suggest that the device presents the best performance at an annealing temperature of 50℃, then the performance decreases with annealing temperature increasing. We find that relatively low temperature annealing is beneficial to the solvent evaporation, and the high temperature annealing is conducible to reducing the coherence length of the π-π stacking, which results in the degradation of the performance of the device.