This paper aims to explore the relationship between corporate governance and firm value of Islamic banks in countries of the Gulf Cooperation Council (GCC), and to investigate the direct effect of the scale of the Sharia Supervisory Board on the firm value. 38 conventional commercial banks and 27 Islamic banks are explored. The period of research lasted from 2003 to 2007. To conduct a comparative analysis on the 65 banks based on the panel data, the Hausman-Taylor analytical model was used. The findings of the analysis predominantly concerns: 1) the absence of a significant relationship between corporate governance and firm value for both Islamic and conventional banks in GCC countries, 2) the absence of a significant relationship between the scale of the Sharia Supervisory Board, which manages and monitors Islamic banks, and bank firm value.