Unethical performance is an impediment for economic development and good governance. It is more than true to say it is the bane of development in developing nations of the world under which Nigeria is categorized. The construction sector was highlighted for its practices in areas such as bribery, environmental destruction, capital flight, dangerous practices, poor quality, whistle blowing, etc. and it is also associated with unethical behaviour through the entire construction life cycle. This paper therefore, x-rayed the determinants of unethical performance in Nigerian construction industry with a view to identifying the causes in all the stages of building project. Well structured questionnaires were self administered to direct stakeholder involved in the execution of building projects considered for this research work. Literature review revealed that corruption is evident in the construction industry which causes a lot of setback to project such as abandonment of such project and if complete may be completed below standard. Data were presented and analyzed using non parametric statistic; tools used were Severity indices and Relative importance index rather than mean scores since the data were ordinal in nature. It was observed that the building construction industry is perceived to be more susceptible to ethical problems because of several features and that corruption has effect on all stages of construction right from Planning, Tender stage to Completion stage. It was recommended that viable legislation as a mechanism to deal with small levels of corruption by strengthening professional institutions that will punish erring members will prevent corruption to a certain degree; and enforcement and monitoring measures of anti corruption agencies is believed can enhance transparency, accountability and reduced unethical behavior, and this will create an enabling environment possible for the industry to thrive, operate, and improve on quality and quantity of infrastructure on a more sustainable basis and thereby foster good construction practice ethics.