In order to study the fracture behavior at the side shell to annular fillet weld toe in the flat bottom oil storage tank, the burst test of model tanks has been carried out by hydraulic internal pressure. It is clarified from the results that the initiation of the ductile crack is the critical event in the fracture of the tanks, and the ductile fracture toughness of materials and the geometry of the weld toe have the significant effect on the fracture. So far, L-shape bend test has been performed and the effect of the geometry of the weld toe and mechanical properties was formulated. In the present study, the fracture criterion of fillet weld toe in the tank is derived by modifying that formula based on the crack initiation characteristics. Finally, the reliability analysis has been carried out by means of Monte-Carlo simulation. The probability distribution for the weld toe geometries is assumed to be normal and Weibull function based on the data measured in the real tanks. The ductile fracture toughness δ _i is evaluated using two and three parameters Weibull distribution functions. The reliability of tanks are disscussed from these results.