One of the major concepts of microeconomics is "market" and its various types. There is a structure, conduct, performance (SCP) paradigm that applies to every market. This study surveys the relationship between return on assets (ROA) as an indicator of performance and minimum efficient scale (MES) and concentration ratio (CR) as two structural elements of the industries active in Tehran Stock Exchange (TSE) based on the International Standard industrial classification (ISIC) during 1379-1384 (2000-2006). In order to estimate the relationship between these variables, Panel data method has been used.Findings indicate that MES and CR collectively had a significant effect on ROA. Furthermore, there is a negative relationship between MES and ROA. The highest ROA was related to "oil, coke, and nuclear fuels" industry and the lowest to "textiles" industry. CR and MES collectively account for more than 59 percent of ROA change in the Iranian industries. MES had a significant effect on ROA in "printing and Publishing", chemical", "other non metallic mineral ores", "basic metals", and "vehicles and their spare parts" industries. Moreover, CR had a significant influence on ROA in "oil, Coke, and nuclear fuels", "rubber and plastic", "food products", "paper products", "printing and publishing", chemical", "electrical machinery", "other non metallic mineral ores", "basic metals", and "machinery and equipment" industries.