Iranian Mental Health Survey (IranMHS) was conducted to assess the twelve-month prevalence and severity of psychiatric disorders in the Iranian adult population and to determine the pattern of health care utilization and cost of services. IranMHS is a cross-sectional national household survey with face-to-face interviews as the main data collection method. The study was carried out between January and June 2011. A three-stage probability sampling was applied for the selection of a representative sample from the non-institutionalized population aged 15 to 64. The primary instrument utilized for assessing the prevalence of mental disorders was the Persian version of Composite International Diagnosis Interview, version 2.1. The instruments for assessing the service and cost of mental illness were developed by the research team. The response rate was 86.2%, and a total of 7886 individuals participated in the study. Sampling weights were the joint product of inverse probability of unit selection, non-response weights and post-stratification weights. This paper presents an overview of the study design, fieldwork organization and procedures, weightings and analysis. The strengths and limitations of the study are also discussed.