This study aims to examine the effect of board composition on firms' leverage and to evaluate the findings in light corporate governance theories and capital structure theories. It is used panel data analysis based on annual data of firms currently listed in BIST Corporate Governance Index over the period 2012 to 2018.The findings of the study reveal that board size, foreign directors and firm size have statistically significant and positive effects on leverage. On the other hand, female directors, independent directors, Chief Executive Officer (CEO) duality and Return on Asset (ROA) have a negative effect on leverage. However, among these factors, only ROA has a statistically significant effect.