OBJECTIVES: To assess the socioeconomic-related (in)equality and horizontal (in)equity in oral healthcare utilization among Thai adults after Universal Coverage (UC) policy implemented nationwide, and to decompose the source of inequality in utilization. Further, to identify the determinants that effect to out-of-pocket payments for oral healthcare. METHODS: Using the data of 32748, Thai adults aged 15 years and over from nationally representative Health and Welfare Survey and Socio-Economic Survey in 2006. This study employs concentration index (CI) and horizontal inequity index (HI) to measure the socioeconomic-related inequality and horizontal inequity in oral healthcare utilization, respectively. Further, employing decomposition method to identify the sources of inequality comprising of a contribution of income, need determinants (i.e. self-assessed oral health, demographic characteristics), non-need determinants (i.e.working status, educational level attainment, type of insurance entitlement, geographic characteristics and marital status) and residual term. Two-part model is used to determine the factors effect to out-of-pocket payments for oral healthcare. RESULTS: There are the pro-rich inequality and inequity in oral healthcare utilization among Thais as indicated by significantly positive values of CI (=0.199) and HI (=0.206). The poor are more likely to access and utilize services at subsidized public facility particularly community hospital, as opposed to the better-off who tend to utilize services at private facility. Income and non-need determinants principally contribute to the pro-poor in public sector utilization, unlike pro-rich in private sector utilization. Need factors account for most of the pro-poor utilization. Type of treatment obtained and insurance used in the last visit are the substantial determinants effect to incurring out-of-pocket payments for oral healthcare. CONCLUSION: Notwithstanding the UC policy implementation made impressive strides toward improving of welfare coverage and an increase in accessibility of health services among Thais, inequality and inequity in oral healthcare utilization persist even when the country achieved universal coverage. Decomposition analyses demonstrate the association of each determinant to inequality in utilization which provides information for policy amendment to achieve the goal of equity in healthcare system.