The high-strength valve spring uses, as the material, a steel containing 0.5-0.8% C, 1.2-2.5 wt % Si, 0.4-0.8 wt % Mn, 0.7-1.0 wt % Cr, balance Fe and inevitable impurities, where, in the inevitable impurities, Al is no more than 0.005 wt % and Ti is no more than 0.005 wt %, and the largest non-metallic inclusion is 15 .mu.m. In the oil tempering treatment, the heating temperature at hardening is between 950-1100.degree. C., and nitriding treatment is performed after coiling. It is preferable to nitride at a temperature no lower than 480.degree. C. Since the material is a high-silicon steel, the tempering temperature can be set at a higher temperature, and the nitriding temperature can be so high. In another way, after coiling, the spring is subjected to shot peening at least twice with shot particles of hardness 720 Hv or higher to produce a compressive residual stress of 85 kgf/mm.sup.2 at around surface. These measures render high surface hardness, and produce high-strength valve springs having good fatigue strength, sag resistance and delayed fracture resistance.