A ceramic isothermal forging die comprises a pair of disk- shaped die halves that are formed of ceramic and have die cavities cut in their mating surfaces and a pair of die half holders each having a recess for accommodating one of the die halves. Each die half is divided into a plurality of radial segments whose bottom surfaces are tapered away from their mating surface with increasing proximity to the center of the die. The floor of each die half holder is tapered to become deeper toward the center of the die and to be complementary to the tapered bottom surfaces of the die segments. The ceramic material of the die makes it usable in the atmosphere. Because of the segmentation of the die halves and the tapering of the segment bottoms and the floor of the die holder recesses, a large force which would otherwise be produced in the circumferential direction of the die by thermal stress during heating and cooling is suppressed. Further, a force urging the segments toward the center of the die is produced during forging. As a result, that die is protected from breakage and from disintegration by separation of the segments.
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