Abstract: On the assumption that EBIT(earnings before interest and tax)is a stochastic variable,whose probability density submits to even distribution,based on the theory of static trade-off,we built models on the relationships among debt and tax shields and the cost of bankruptcy.Finally,in a same debt scope we synthesized tax shields and the cost of bankruptcy into models and analyzed net tax shields and drew a conclusion that there existed optimal debt and thus optimal capital structure,given a group of firm’s parameters.

Key words: theory of static trade-off, tax shields, the cost of bankruptcy, even distribution