In order to investigate the effects of non-metallic inclusions on the strength of steels, the behavior of non-metallic inclusions under tensile stress was studied by microscopic observations and the fatigue properties of several 3%Ni-Mo-V steels made by different processes were tested. The results were as follows: (i) At a slight excess load over the yielding point, microcracks are found at the corners of non-metallic inclusions, especially on the transverse specimen; (ii) between the elongated type inclusions, silicate is more harmful than sulfide to transverse fatigue properties; and this is borne out by the higher strain concentration evident at the silicate during tensile test; (iii) the effect of sulfide on fatigue properties does not exceed 10%, even though the sulfide content is rather high and vacuum-poured electric-arc furnace steel has good cleanliness and good fatigue properties; and (iv) the notched fatigue strength shows little anisotropy.