Abstract：Some scholars have discussed extensively the interaction of capital structure, investment and product market behaviors. They suggest that debt leads to lower investment expenditures and weaker or stronger product market competition. They, however, look only at two of the above behaviors. This paper develops a model which examines all three behaviors and shows that debt and investment can be substitutes in a model where firms rationally take on debt. Furthermore, it is demonstrated that when firms compete with prices in the product market, an increase in debt leads to lower investment and higher prices. Key Words：Debt Investment Product Market Behavior Limited Liability Effect Credible Commitment