This paper proposes an effective blind spot detection and warning system in the daytime and nighttime condition. In the daytime condition, the proposed system presents the Horizontal Edge and Shadow Composite Region (HESCR) method to extract the searching region and to acquire the shadow location of the targeted vehicles. Additionally, to overcome the nighttime scene, the proposed system extracts the bright objects and recognizes the paired headlights of the targeted vehicles for the BSD. The results of the proposed system are yielded by the practical experiments on the highway in Taiwan at the nighttime and daytime conditions. Experimental results show that the proposed BSD system is feasible and effective for vehicle detection and identification in the daytime and nighttime environments.