This paper mainly has two objectives: the first one is to identify the similarity of capital structure between Islamic and Conventional banks; Second objective is to detect the relationship between capital structure variables and performance of Islamic and Conventional Banks in Gulf Countries (GC). This investigation has been performed on a sample of 16 GC Banks (8 Islamic Banks and 8 Conventional Banks) for the period 2005-2014. ROE (return on equity) and ROA (return of asset) have been used as performance measures. Total debt to total assets, Equity to total assets, Debt to equity ratios have been used as capital structure measures. Size of the bank has been considered as dependent variable to identify its relationship with bank performance. Data collected were analyzed by using SPSS software. The results of the research indicate a similarity of capital structure of Islamic banks and Conventional banks in Gulf Countries. ROA as performance measurement has a significant negative relationship with financial leverage and a positive relationship with equity to assets ratio. This relationship is identified at Islamic banks, Conventional banks and all the banks of the sample. Bank size has a positive relationship with ROA and ROE as performance measures in Islamic and Conventional banks. modifications on the market of 122 equity indices and 39 commodities in the eight approaches, depending on the investment time horizon (October-15^th May, November-15^th May, October-1^st May, November-1^stMay) and types of computed rates of return (accrued rates of return and average daily geometric rates of return). Calculations presented in this paper indicate the presence of the sell-in-May-and-go-away effect on the analyzed markets in the classic time frame, as well as in the different time frames. ation in the country. Markets determine nominal exchange rate should prevail in the economy. The country should regulate its foreign reserve policy by setting a threshold, above which excess deposit should be plough back to the domestic economy inform of investments rather than support excessive importation.