Three kinds of fatigue tests were carried out in low cycle fatigue range on A 5083 P-O aluminum alloy and its butt welded joints in order to obtain basic data for low cycle fatigue designing of LNG tanks. The hour-glass shaped base metal specimens, which were cut out in longitudinal and long transverse direction, were used for diametral strain controlled and repeated strain cycling tests to investigate the effect of strain controlling direction on fatigue strength. Transverse butt welded joint specimens welded by DCSP-TIG, high current MIG and conventional MIG arc welding were also used for either tests of displacement controlled axial load cycling or deflection controlled bending load cycling, changing experimental parameters for reinforcement shape : (1) ground partly, (2) flushed out and (3) as welded conditions. The main conclusions obtained are as follows : (1) Diametral strain controlled fatigue strength of base metal specimen whose direction is longitudinal was influenced with controlling direction of diametral strain in low cycle fatigue range, while that of the long transverse directional specimen did not almost depend on controlling direction. (2) Under axial loading condition, it was found that, regardless of the welding process, the fatigue strength of butt welded joints without reinforcement were almost the same. But difference between the bending fatigue strengths of the butt welded joints made by two kinds of MIG arc welding was observed. Comparison of the bending fatigue strength with axial fatigue strength showed that the former is 1.1 to 1.5 times greater than the latter in the range of this investigation. (3) The fatigue strength of the butt welded joints decreases markedly due to the reinforcement of the weld in both loading conditions of axial and bending. The fatigue strength reduction factor of the butt welded joints made by MIG arc welding will become 2 or more, but by grinding out partly the reinforcement toes of the weld it can be made to increase the fatigue strength up to about 1.5 times.