A methodology to model the weld between the constituent bodies of a bimetallic configuration is presented for estimating the effect of strength gradient across the weld interface on crack growth characteristics under fatigue load. The constituent bodies and the interface weld have similar elastic and thermal expansion properties but different plastic properties. A Mode I crack is in parent soft alloy steel (ASTM 4340) which grows perpendicularly towards the ultra strong weld of maraging steel (MDN 250). A cyclic plastic or cohesive zone develops ahead of the crack tip under fatigue load. This zone penetrates into the weld, possibly crossing over into maraging steel, as the crack grows nearer to the weld which causes the drop in stress intensity parameter at the crack tip from the applied value. The cohesive zone split across the weld interface is modeled in linear elastic regime with the help of Kolosov-Muskhelishvili complex potentials leading to a computational model for assessing the effect of the weld on the advancing crack. Paris law based on cyclic crack tip stress intensity parameter is used to estimate the crack growth rate. Theoretical assessment is substantiated with the help of experiments. Bimetallic compact tension specimens, prepared by electron beam welding of the steels, are subjected to high cycle fatigue load to measure crack growth rates in them. Results of the bimetallic specimens are analyzed and compared with those of the plain alloy steel specimen. Influence of ultra high strength weld substantially reduces the growth rate of the approaching crack resulting in enhanced fatigue life of the bimetallic body. Experimental results are in good agreement with theoretical estimations.