Stiffened plate structure is adopted to tank structure for IMO type B LNG carrier. Fatigue tests were conducted on various types of welded joint specimens and the fatigue properties of the welded elements were investigated. And the results were used to assess the fatigue strength of the structural elements of the tank. Through the course of analysis, fatigue life of the welded joints with respect to stress cycling was found to form a small scatter band when the local stress at location of crack initiation is obtained and standardized to a given stress ratio taking account of the yield stress of the material and the welding residual stress. A rational S - N curve for fatigue assessment is proposed for the local stress amplitude considering the variance of the experimental data. The fatigue crack growth rate and the threshold stress intensity factor range were also investigated to see the effect of stress ratio R with an emphasis on a high stress ratio behavior. Tests were carried out at room temperature and cryogenic temperature (-196°C). The analysis of experimental data yielded that for 0≤ R ≤0.5 the base metal showed a stress ratio dependency whereas the weld metal had small effect of the stress ratio and the crack growth rate was slightly higher as compared to the base metal. But the fatigue crack growth rate for R >0.5 the dependency of R becomes small and the crack growth for the two materials are more or less identical. The fatigue crack growth rate was appreciably low at cryogenic temperature indicating that the materials have a higher resistance to crack extension at low temperature. For the basis of design and safety assessment of the tank structure, an expression is proposed which is an upper envelope curve for da / dN vs. ΔK for high R tests conducted at room temperature. Full scale model of a cutout section of the tank namely, the bottom transverse section and frame end bracket section were fabricated and fatigue tested. It was confirmed that the fatigue strength and the crack growth of all the full scale models were conservative when they are compared with the proposed S - N curve and the fatigue crack growth curve for fatigue assessment obtained from small specimens. A general approach for the safety assessment of the tank for the LNG carrier is also discussed.