In this study, the data of 132 businesses operating in the manufacturing sector in the BIST 100 index were analyzed by econometric methods to test the validity of the theory of pecking order theory and trade off theory, which are included in the literature as capital structure theories. In this context, panel data analysis was used to test the validity of capital structures. The study covers the years 2006-2016. The dependent variables of the study are determined as the ratio of short-term debt to total assets, the ratio of long-term debt to total assets, total debt ratio and the independent variables are determined as the liquidity ratio, the asset structure of the company, the profitability ratios, the size of the firm, the non-debt tax, and the tax level ratio. As a result of the study, it was determined that there was no statistically significant relationship between firm size, return on assets and tax rates and financial leverage ratios in all the established models. BIST 100 companies operating in the manufacturing sector have reached the conclusion that pecking order theory is valid.