Using three waves of Thailand Household Socio-Economic Surveys in 2006, 2011, and 2017, this paper analyzes the inequality of opportunity trend over time and its contribution to total wealth inequality in Thailand. The empirical results based on non-parametric and parametric techniques demonstrate the declining inequality of opportunity in the overall inequality from 2006 to 2017. The lower bound estimation of inequality of opportunity caused by external factors beyond an individual’s control (i.e., circumstances) accounted for 16–17% of overall inequality in 2006, compared with only 11–14% in 2017. Based on our analysis, higher education could be one of the crucial elements in narrowing the differences in individuals’ wealth.