Experimental studies were carried out to examine the influences of flank angle and undercuts at the weld toe on the fatigue strength. Non-load carrying cruciform and tee fillet welded joints were tested and quantitative life reduction due to undercut was obtained. It was also found that the flank angle has a little influence on the fatigue strength. The lives of specimens to failure were analysed based on the stress concentration factors at the weld toe. Visible crack initiation life was evaluated by calculating the elastic stress cyclings at the weld toe and crack growth was estimated through the Paris' law. The specimen life to failure was estimated s the sum of these two lives. Estimated lives were compared with test results and it was confirmed that this procedure gives the fatigue life of the fillet weld specimen with sufficient accuracy. The influence of undercut depth could quantitatively be discussed based on the estimated life of welded joints as well as test results.