Abstract: Teeth of saurischian tyrannosaurid and squamate mosasaurid are studied mainly by scanning electron microscopy. The teeth of both diapsids are cone —shaped homodontic teeth mediolaterally compressed and curved posteriorly. The sharp cutting edges at both anterior' and posterior aspects of tyrannosaurid tooth are serrated with invagination of a quite thin aprismatic enamel layer. Fissures and pits of the invaginations are packed with what is possibly a layer of fossilized organic substance. In contrast, similar serrated cutting edges are only observed at the basal portion or lepidosaurian mosasaurid teeth. The present scanning electron microscopy (SENI) observes a. similar intermediate type orthodentine (tubular dentine) with ill —developed peritubular dentine of both extinct reptiles. SEM reveals that dentinal tubules are regularly branched only in the vicinity of the flat dentino —enamel junction in tyrannosaurid tubular dentine, bul are complicatedly branched in the rnid and superficial —layer of mosasaurid dentine. The ryesent study also shows a few dentinal tubules invaded the enamel layer, with a similar configuration and an evident distribution of interglobular dentine beneath the thin enamel layer of both reptiles. Based on the present fine structure study, we elucidates some histological similarities and dissimilarities of the enamer and dentine in the reptilian tyrannosaurid and mosasaurid of different infraclasses.