A spring steel having a good sag-resistance and a good hardenability comprises by weight 0.5-0.8% carbon, 1.5-2.5% silicon, 1.6-2.5% manganese and a member or members selected from a group consisting of 0.05-0.5% vanadium, 0.05-0.5 niobium and 0.05-0.5% molybdenum, the remainder being iron together with impurities. The steel may further contain a member or members selected from a group consisting 0.0005-0.01% boron, 0.2-1.0% chromium, 0.2-2.0% nickel and not greater than 0.3% rare-earth elements and/or a member or members selected from a group consisting of 0.03-0.1% aluminum, 0.02-0.1% titanium and 0.02-0.1% zirconium.