A mathematical model has been developed to predict the fatigue life of Flux Cored Arc Welded (FCAW) cruciform joints containing Lack of Penetration (LOP) defect. High strength, Quenched and Tempered steel ASTM 517 'F' Grade has been used as the base material throughout the investigation. Two level, full fac­torial technique has been applied to design the experiments, The model has been developed by Response Surface Method (RSM), The adequacy of the model have been checked by ANOV A technique and corre­lation co-efficient. By using the developed model, fatigue life of flux cored arc welded cruciform joints. con­taining LOP defects, can be predicted at 95% confidence level.