Abstract: Direct observation was accomplished on the fatigue crack growth behaviour in 16MnR steel weld HAZ with SEM and the fatigue loading equipment. Based on the observed results, an intermittent-kinked-forked fatigue crack growth model was developed and a calculating formulation for fatigue crack growth was established. The formula incorperates local strecs intensity factor, mechanical, cyclic and fatigue properties as well as a damage length parameter. The predicted values of the model were compared with the experimental data and they are in a good agreement.