Abstract: Prediction of microstructure and hardness in HAZ is very important for selecting the optimum welding parameters. The common methods are often relied on SHCCT curves, having a narrow predictable field, and being complicated in application. In present paper, the kinetics theory of phase transformation is applied to welding process. Formulae for calculating the microstructure and hardness of microalloyed steel are given. The calculating process is formulated and is convenient to use on microcomputer. Unknown constants are calibrated by experiment data from actual welds. Two steels are used to check the model. The result shows that there is a good agreement between calculated and measured values.