The study examined the mitigating measures employed by contractors in southwest of Nigeria for cost overrun of building projects with a view to determining its adequacy and effectiveness. A mix of qualitative and quantitative research methods was adopted where data pertinent to the study were obtained through questionnaire survey on a sample of 32 project managers of medium sized construction firms selected from the list of 125 contractors on the register of Federation of Building and Civil Engineering Contractors located in South-West Nigeria using Simple Random Sampling method. Additional information was obtained from contracts bills of quantities and programme schedule to complement the data obtained from the questionnaire survey. Contract sums and final sums of building projects between 200 million Naira – 1.7 billion Naira handled by respondents were also collected as secondary data. Statistical Package for Social Scientists (SPSS) was employed to analyse the data for descriptive statistics while manual approach was adopted for inferential statistics of t-test. The results obtained revealed that effective site management and supervision with the highest relative importance index (RII) for adequacy of the mitigation measures was at average level and the (RII) for the effectiveness of mitigation measures adopted by contractors was very low. It also revealed that there is significant difference between initial sums and final sums of building projects at 5% significance level where the mitigation measures were taken in to consideration. The study concluded that mitigation measures adopted by contractors have not been adequate and effective in curbing cost overrun of building projects in southwest Nigeria.