The existence of governance structure such as board of commissioners and audit committee implies a better monitoring function on the financial reporting process that will result in higher informativeness of earnings. Nevertheless, the quality of the monitoring function depends on how effective the board and the audit committee perform their duties. This study examines how the effectiveness of the board of commissioners and audit committee in a two-tier board structure influences the informativeness of earnings which is measured by Earnings Response Coefficient (ERC). The level of effectiveness is defined as the extent to which the board of commissioners and the audit committee perform their duties according to their responsibilities, which is associated with the board and audit committee characteristics. This study develops a score for board and audit committee effectiveness by using a checklist which captures how the independence, activity, size, and competence characteristics are reflected in the board of commissioners and audit committee. Previous studies find that these characteristics are required to enable the board and audit committee perform better governance. Each question in the checklist will be scored 1 (Poor), 2 (Fair), or 3 (Good) based on the information disclosed in company annual reports. Hypothesis testing is carried out by using a multiple regression model of 357 observations (firm-year) with the sample taken from companies listed on the Indonesian Stock Exchange during the period 2006-2007. Results of this study provide robust evidence that a more effective board of commissioners improves the informativeness of earnings. However, the results for the influence of the audit committee effectiveness are still mixed. The findings indicate that the effectiveness of the audit committee does not affect the informativeness of earnings, but further analysis indicates that audit committee effectiveness has a positive effect on ERC when the board of commissioners is not effective in performing its role. Based on these findings, this study indicates that audit committee is an integral part of board of commissioners so that the role of board of commissioners still have significant role in the monitoring function because the audit committee actually act on behalf of and report to the board of commissioners. Therefore, the role of audit committee will be more revealed when the board itself is actually less effective. This is consistent with the objective of the audit committee formation which is to provide a complementary function for the board of commissioners monitoring role.