This study examines the influence of corporate tax aggressiveness on corporate debt policy (the debt-substitution effect) and the influence of outside directors on both debt and the debt-substitution effect. Based on a sample of 6967 firm-year observations over the 2001–2010 period, we find that tax aggressiveness is negatively correlated with debt. We also observe a negative correlation between debt and the proportion of outside directors on the board, and find that outside directors magnify the debt-substitution effect. Finally, we obtain similar results in analysis based on firms' debt issuance decisions.