The main purpose of this paper is to analyze the influence of debt changes on enterprises value at Warsaw Stock Exchange between 2005-2010. Several capital structure theories were presented in the first part of the paper. In the second part the empirical model was constructed based on the company and market specific data obtained from the companies financial statements and stock quotations. The most important conclusion is that the increases in companies` debt causes decreases in their market value during the time of global financial crisis. Positive impact on companies valuation have also the net earnings and market behavior.