The study focused on critical assessment of the Nigeria police and national security in Nigeria's fourth Republic. Specifically, the paper examined the constitutional role of the police as an important agency of the state that plays a key role in the enhancement of national security. No doubt, the Nigeria police Force in several instances have been playing some roles that have been contributing to national security in Nigeria. Over the years however, the unwholesome activities of some men of the police command have continued to leave sad memories in the mind of the people. Against this background, the paper examined some of the activities of the Police command that have constituted serious threats to Nigeria's national security in the Fourth Republic. The paper employed qualitative method through the use of secondary (library) and primary (interview) sources of data collection. The data were descriptively analyzed through detailed information gathered from documentary sources and interviews. The Marxian approach to the theory of post colonial state in Africa was found suitable in the analysis. The paper finds that despite current reforms to rightly reposition and refocus the Nigeria Police Force, some members of the command have continued to indulge in acts that have continued to constitute serious threats to national security in Nigeria. The study recommended among others; better equipping of the Nigeria police and adherence to professionalism by officers and men of the police command.