Abstract : Financial reports are the primary communication channel between management and external stakeholders. Reported earnings quality is an area of concern, as managers can manipulate final results through the authority they are granted by firms' owners to manage firms' day-to-day business. Income smoothing is one type of earnings management used by managers to manipulate reported earnings. It is an attempt to reduce the volatility of income, wherein the net income is inflated or deflated so that income movement seems smoother and has less fluctuation (Ibrahim et al., 2020). Corporate governance is a method that prevents management from misusing their authority (Yanti & Dwirandra, 2019). It helps in aligning the interests of managers with those of shareholders and enhances the reliability of financial information and the integrity of the financial reporting process (Watts & Zimmerman, 1986). So how effective are corporate governance mechanisms in preventing income-smoothing behaviour? Does firm value as an indicator of shareholders' wealth and firms' future performance motivate managers to smooth the individual firm's income? To answer these questions, data for non-financial listed firms in GCC markets were collected from 2011 to 2022, and a total observation of 3,278 datasets for 303 firms were analyzed by the application of ordinary least square and multivariate regression models. Results showed that 93% of total observation practiced income smoothing. Only board size and firm value significantly influenced income smoothing, while there was no relationship between other variables (independent directors, CEO-chairman duality, meeting frequency, audit committee and financial expertise) and income smoothing. The study model was significant and may predict income-smoothing behaviour. These results showed that the current corporate governance laws are ineffective in controlling management attempts to report more stable income. Regulators should review and update current laws to protect the rights of shareholders and other stockholders. This was derived from the need for GCC countries to diversify their economic resources and draw in foreign direct investment, especially with the growth of financial markets brought on by globalisation and unstable oil prices. Keywords: income smoothing, corporate governance, board of directors, audit committee, GCC countries.