The thickness effect of fatigue strength was investigated on welded steel joints with thicknesses ranging from 10 to 80 mm. As-welded cruciform joints and Tee joints were tested under pulsating tension and pulsating bending, respectively. The combination of main plate thickness and welded attachment size was varied systematically. As a result, it was revealed that thickness effect is clearly influenced by the combination of main plate thickness and attachment size. A series of joints with attachment proportional to main plate thickness showed the greatest thickness effect. Another series of joints with constant main plate thickness and increasing attachment size was second. It was followed by the series of joints with constant attachment size. As for the other influencing factors, pulsating bending has shown a greater thickness effect than pulsating tension. This means that the thickness effect in proportional joints under bending stress is very large. The thickness effect for crack initiation was largest, and it was followed by failure life and then by crack propagation life. This was confirmed by finite element analysis. It was also clarified that the longer the fatigue life, the greater the thickness effect.