This paper investigates the inequality of opportunity in China's labor earnings, defined as the component of inequality determined by personal circumstances that lie beyond the control of an individual, of which gender is one, as opposed to the component determined by personal efforts. Using the Survey of Women's Social Status in China (2010), we measure the share of inequality of opportunity in the total inequality of individual labor earnings for people aged 26-55 years, and separately for six birth cohorts and for female and male subsamples. Gender is revealed as the single most important circumstance determining nationwide individual labor earnings, with one's region of residence, father's occupation, father's education, birth cohort and holding rural or urban hukou also playing significant roles. A further investigation into the roles of circumstances and personal efforts (including education level, occupation, Communist Party membership, migration and marital status) confirms that circumstances play an alarmingly high role in shaping labor earnings distribution in China, and reveals notable gender differences that cannot be attributed to personal effort alone. These results provide the basis for recommending ways to improve gender equality of opportunity in the future.