To make clear the pretwisting effect on the strength of a helical spring, its static and fatigue properties were examined on several kinds of helical springs made of wire twisted ordinarily and inversely during or before the coiling.The results obtained are as follows: (1) The effect of pretwisting on the strength of a helical spring depends on the mode of twisting A remarkable improvement can be obtained by ordinary twisting during the coiling and a slight improvent by twisting before the coiling. (2) By inverse twisting the fatigue strength of a spring decreases remarkably as compared with the elastic limit because of tensile stresses acting on the crack plane produced by pretwisting of the wire. (3) By prestressing or so-called setting process a fair increase in the elestic limit of a spring can be obtained, but the fatigue limit is not much increased.