Located in Northeastern Arabia, Thaj is the largest known pre-Islamic site in the region where urban and sub-urban areas are associated with a vast necropolis of more than 1000 tumuli. As part of the Franco-Saudi mission of Thaj (CNRS/Saudi Arabia Ministry of Culture), the aims of the research conducted on the necropolis since 2017 was to characterize the funerary practices and to understand the conditions of development of the necropolis from the Iron Age to Late Antiquity. This paper presents some of the results of the study carried out through a multidisciplinary approach, based on aerial and ground surveys, the excavation of several tombs and the anthropological analysis of the human remains. The field data allowed the observation of different tumuli construction techniques and the establishment of the first typology of the Thaj tombs. Inside the tombs, one to several dozen burials were discovered whose architectural diversity is partly dependent on the age at death. The differences in the treatment of the deceased also seem to highlight the existence of a social stratification of the population. Combined with radiocarbon dating, it was possible to distinguish several chronological phases in the Thaj necropolis in relation to the evolution of the funerary architecture.