This study is undertaken to explore levels and trends of under-five mortality rates (U5MR) at the national level among general Thais and 9 hilltribes as well as compare the rates across socioeconomic strata within and between ethnic groups by making use of information on the 2000 census. The Trussell version of the Brass indirect method was employed to estimate U5MR. The gap of mortality inequality was estimated by both absolute and relative difference. The findings indicated that U5MR in all hilltribes were comparatively higher than Thais over time. Yet, the mortality differentials between them tended to shrink eventually. The variations of U5MR among studied ethnic groups rely on their SES: Thais with a large proportion in high SES were more likely to enjoy low death rates than their counterparts. Further, inequalities in child mortality evidently found within ethnic group; the rates often fell in a continuous linear gradient from the lowest to the highest SES. The socioeconomic differences in child mortality were relatively large among hilltribes, but they were small among Thais. These mortality disparities between Thais and hilltribes require policy attention to reduce excess deaths by focusing on both social and economic disadvantages faced by hilltribe families.