During the service of steel bridges, typical fatigue cracks have been found at the toes and the roots of welded joints considered as load-carrying fillet welded joints. It is well known that the fatigue failure modes are influenced by three fundamental factors: the weld size, the weld penetration and the plate thickness. However, since their effects on fatigue failure mode have not been clarified, no evaluation method has been established. In this study, parametric fatigue life analyses were carried out on the fillet welded cruciform joints in order to quantitatively clarify the influences on the fatigue failure modes. According to the analytical results, a quantitative method for evaluating the fatigue failure modes was proposed in consideration of the weld size, the weld penetration and the plate thickness.