Given the great importance of fraud risk assessment as a part of audit process, investigation of auditors’ individual effectiveness and its effect on the fraud risk assessment error is an essentially important subject to consider. Individual effectiveness implies the individual's performance in the organization, how the individual perform his/her task, and the level of goal achievement by the individual and fraud risk assessment relates to achieve the reasonable assurance by the auditor based on that the financial statements are free from material misstatement caused by mistake and fraud. In terms of nature, this study is applied and In terms of objectives is descriptive-survey study. Theoretical foundations of the research were collected through library studies and research data were gathered by the questionnaire. The study sample consists of 518 private and public sector auditors in 2017 and the sample was selected by Clustering random sampling. The research hypotheses were examined based on the correlation method and through multivariate regression test. The findings obtained from the current study indicate that in both public and private sectors, there is a significant and positive relationship between auditor's individual effectiveness and fraud risk assessment error. In the public sector, the fraud risk assessment error is more than private sector while in the private sector, auditor's individual effectiveness is in higher level than the public sector. The results of this study indicate that gender has a significant relationship with the auditor's individual effectiveness in both public and private sectors. Education have not a significant relationship with the auditor's individual effectiveness in both public and private sectors. Also, auditors' experience and job rank have a significant relationship with their individual effectiveness in the private sector, but this finding is not significant in the public sector.