This study aims to examine the effect of voluntary assurance of corporate social responsibility (CSR) report on the cost of equity capital. Using 2014-2017 Taiwanese listed companies as the sample, the results show that assurance helps to improve the reliability of CSR report and the trust of investors, thereby reducing the information risk assumed by investors when buying stocks. Therefore, investors are willing to lower the rates of return required by them, so that the cost of equity capital of companies will be reduced about 0.55%. This result is mainly driven by the independent third parties which are consultant firms instead of accountant firms.