Completely reversed bending fatigue tests were carried out on fillet welded cruciform joints with no penetration of mild steel and the effects of the ratio of a fillet size, h, to a main plate thickness, t, on their fatigue strength were examined with the aim of fracture mechanics method. The main conclusions obtained are as follows : (1) It can be assumed that the fatigue crack initiation life, Nc, is equal to the fatigue failure life, Nf, in a toe of weldment since the thickness of the main plate is thin on fillet welded cruciform joints used in this study. The Nf at the toe failure may be estimated using the relation between Nf and a stress-strain function, D, obtained from the results of reversed bending fatigue test conducted by the smooth plane specimens of a base metal.(2) In the reversed bending fatigue tests, a weld failure occurs at any stress amplitude on the fillet welded cruciform joints with h/t of below 0.3, while a toe failure takes place on those with h/t of over 0.5. On the fillet welded cruciform joints with h/t of 0.3 to 0.5, which correspond to critical weld size, the weld failure occurs at higher stress ranges, and the toe failure at lower stress ranges.(3) The fatigue crack growths from the tip of a lack of penetration are observed with optical microscope when the variations of stress intensity factor, ΔK, which are obtained by identifying the lack of penetration as an initial crack, are more than the variation of threshold stress intensity factor, ΔKth, in a weld metal.