This study aimed to examine the extent to which income-related inequality and horizontal inequity in outpatient and inpatient care utilization among Thais are manifest after the country implemented the Universal Coverage (UC) policy, by using a concentration index and a horizontal inequity index, respectively. Furthermore, the study examined the determinants and their associations with the observed inequality, if any, in health care utilization through decomposition methods. The nationally representative Health and Welfare Survey 2005 was used to perform the analyses. Although there are socioeconomic gradients in health care utilization among Thais, the findings reveal that health care utilization tends to favor the poor in particular with utilization at the public facility and especially at the primary care level facility. Thailand has made impressive strides toward nearly universal health insurance coverage and improving access to and utilization of health care for its population, particularly among the poor.