Good Corporate Governance (GCG) is one alternative to eliminate earnings management (as a proxy for the usefulness of accounting information). The existence of GCG elements is expected to improve the quality of GCG implementation. The study is based on three grand theories, namely agency theory, signaling theory, and positive accounting theory. The objectives of the study were to analyze: (1) the impact of corporate governance structure and (2) the effect of the political cost (asset size as proxy) on earnings management, (3) the impact of earnings management on the earnings response coefficient (ERC). The samples of the study were 122 public firms from the non-financial industry listed on the Indonesian Stock Exchange in 2010-2014. The research data were processed by the Statistical Package for the Social Sciences (SPSS). The result of the study was that institutional ownership, management ownership, and the proportion of the board independent commissioners were variables that significantly influenced on usefulness of accounting information that was proxied by earnings management. The total number of board commissioners and asset size were variables that do not affect earnings management. Meanwhile, the main finding of the study was that earnings management as a signal could be responded to by markets (investor or potential investor), which showed that earnings management affects ERC