Abstract: In this research, a transient microstructure-prediction program was designed on the basis of coupling a previous heat transfer program. With the calculation eguations for phase change point in the phase diagram and transformation kinetics, it can provide the volume fraction of each kind of possible microstructure at any location in the whole welded joint plus iso-volumetric fraction contours. In comparison with the measured values, the relative difference is about 10%. With this program a practical example, in which a circumferential weld was performed with SMAW,was computed. The material concerned was a low-alloy steel used for pipeling construction. This research verified the realizability and reliability of the program.