Fatigue properties of CO2 laser welds in A5182P-O alloy were evaluated by the fatigue test, and the results were compared with those of the base metal and electron beam welds. The initiation and propagation of a fatigue crack during testing was investigated by observing the specimen surface through CCD video camera and by measuring strains with the gauges. Characteristics of fatigue crack surfaces were ana1yzed and confirmed by SEM observation of the specimens. Especially, porosity at the cross section of a specimen before the test was observed by X-ray CT method, and the influence of the porosity on the fatigue strength and cycles to fracture was evaluated in terms of the maximum porosity-occupation percentage per cross section. It was confirmed that the porosity in laser weld metals reduced the fatigue strength and cycles to fracture. It was also found that the maximum size and the percentage of the porosity should be decreased to the degrees of less than 0.5 mm and 1％, respectively, in order to produce the laser welds with good properties equivalent to those of the A5182 base alloy.