The possibility of grain refinement by thermal cycling is examined on commercial heat-resistant alloys, namely, an austenitic 21 Cr-4Ni-9Mn steel and a cobalt-base HS-21 alloy, in which the grain-boundary reaction occurs. A thermal cycle is composed of a high-temperature ageing which causes the grain-boundary reaction, and a subsequent short-term heating at the resolution temperature, which ensures complete dissolution of the grain-boundary reaction precipitates into the matrix. The grain diameter is finally reduced to about one-half or one-third of the original grain size after four thermal cycles, while a larger grain-size reduction is observed in the specimens with initially larger grain size. The effects of the amount of the grain-boundary reaction and the heat-treatment conditions on grain refinement are also experimentally discussed.