Abstract: Welded joints were simplified as sandwiched crack bodies of CCP type.By means of experiments and elastic-plastic finite element method,the law of fatigue crack propagation in the simplified models both with and without mechanical heterogeneity was researched.It is found that,fatigue crack growth not only depends on material properties of crack tip region,but also on material properties adjacent to the crack tip seriously.The shape of plastic zone and distribution of stress and strain in heterogeneous cracked body magnificently differ from these of homogeneous one.It can be considered that the essential effect of mechanical heterogeneity on the fatigue crack growth life is its effect on crack closure.