THE Tyrannosauridae were the dominant large carnivorous dinosaurs in Asia (excluding India) and western North America during the Late Cretaceous period(1-3). Most of them are from the Campanian and Maastrichtian ages, and very little is known about their earlier history, although scanty remains have been reported from the early part of the Upper Cretaceous(4-7). We describe a newly discovered incomplete skeleton of a large theropod from the Early Cretaceous Sao Khua Formation of northeastern Thailand as an early and primitive representative of the Tyrannosauridae. This new taxon, which is at least 20 million years older than the earliest previously known tyrannosaurids, suggests that the early evolution of tyrannosaurids may have taken place in Asia.