The effect of heat input with 20, 50 and 80 kJ/cm on the microstructural evolution of the simulated heat affected zone (HAZ) has been studied in a modified 2.25Cr-1Mo steel. The microstructures of simulated coarse-grained HAZ has been examined by optical metallography and transmission electron microscopy. It was found that large amounts of martensite with small quantities of bainite exist in the specimen with 20 kJ/cm. However, significant amount of bainite with a few amounts of allotriomorphic ferrite can be detected in the specimen with 50 kJ/cm heat input. In the case of heat input with 80 kJ/cm, the simulated HAZ microstructures were composed chiefly of bainite with a few amounts of martensite and allotriomorphic ferrite. In order to study the sequences of carbide transformation, the HAZ specimens were tempered at 700degreesC for different intervals (1, 2, 5, 10, 20, 50, 100, 200 and 500 h). The sequence of carbide transformation in the HAZ zone of this modified 2.25Cr-1Mo steel has been proposed. (C) 2003 Kluwer Academic Publishers.