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PT Ikhtiar Bangun Nusa or PT IBN is a private company that works as service company for drilling and oil field exploration. As a growing company, it will be very useful to know type of financing sources that can maximize their firm value. Author chooses PT IBN because their good prospect in this industry with limited number of service companies that can operate in Indonesia, and increasing world demand of oil. By identifying the optimal capital structure, author expects that it would help company to expand. Author uses two valuation models to generate company's optimal capital structure, Adjusted Present Value Method (APV) and Weighted Average Cost of Capital (WACC). APV method can be used to find company's optimal capital structure by considered the tax benefits from interest payment, present value of bankruptcy cost, and probability default of a company. WACC measures the optimal capital structure by calculate debt and equity proportion of a company to find its minimum cost of capital and maximize the firm value. In the application, APV is more practical on evaluating feasibility additional amount of debt, while WACC easier to use for analyzing debt proportion itself. After generate firm value for each debt proportion, author can estimate debt level which optimize company value. Optimal capital structure result for year 2006 and 2007 is 0% debt, or 100% equity, because company’s negative EBIT. For year 2008, APV method gave 40% debt ratio as the optimal capital structure, while WACC method gave 0% of debt result to optimize the firm value. Different result from these method occur because high expenditure for investment in year 2008, that give negative free cash flow on WACC calculation. In conclusion, with negative EBIT, additional debt ratio will lead company to lower firm value as it shown in year 2006 and 2007. Besides that, for a growing company like PT ABC, WACC method is not suggested to use because their high investment expenditures viewed as cash outflow that can subtract the firm value.