Abstract: The purpose of the present study was to prove that the WACC can be used as a benchmark of capital structure optimization. Population was IDX-listed manufacturing companies from 2009-2013. Sampling using a purposive sampling method. Secondary data were used with a quantitative approach. Hypotheses were tested by using a multiple regression analysis. Analysis showed that the DAR, LDER and the NPM simultaneously had significant effects on the cost of capital. Partial analysis showed that the DAR had an effect on the cost of capital. The LDER and NPM partially had no effect on the cost of capital. Comparison of means showed that ROI was lower than the means of the WACC. Thus, it can be concluded that the WACC can be used as a benchmark of capital structure optimization with the WACC as the minimum ROI.