This paper analyzes Iran's banking market structure, using unbalanced panel data models during 1997-2009. To study the market structure, various measures of market concentration are examined theoretically and the U index of concentration is applied to Iran's banking system. The results show that, along with the commencement of privatization in Iran's banking sector (2001), U index of concentration has tended to decrease, indicating that competition in Iran's banking market has intensified. However, this may not happen until the strict regulations on private banks are relaxed.