Time overruns are major problems facing the Nigerian construction industry. It’s of high concern to those who are involved in the construction industry. This study was carried out to identify the major causes of time overruns in the Nigerian building construction industry, by means of a literature review and a questionnaire survey. A total of twenty (20) time overrun causative factors were obtained from the literature. The questionnaire survey was distributed to randomly selected respondents from a combination of clients, consultants, contractors, site engineers, project managers and sub-contractors. In all, one hundred and forty-one (141) questionnaires were distributed to randomly selected respondents (clients, consultants, contractors, site-engineers, project-managers and sub-contractors), one hundred and thirty-two (132) questionnaires were returned out of which three (3) questionnaires were found incomplete and invalid. Only one hundred and twenty-nine (129) questionnaires were found consistent and valid for use in this research. Relative Importance Index (RII) and Severity Index were used to carry out a ranking analysis. Based on the data received, the five (5) most severe factors influencing project handling overtime in Nigeria construction industries are Inaccurate evaluation of projects time/duration (91.9%), Risk and uncertainty associated with projects (91.6%), Complexity of works (87.6%), Weak regulation and control (86.8%) and Lack of financial power with severity (86.3%).