PURPOSE:To improve the fatigue strength of the titled steel by specifying the contents of Mo, Cr and Mn in a steel and furthermore adding optimum amounts of Nb and V to the steel, thereby activating a working heat treatment. CONSTITUTION:The steel for a spring is constituted of the compsn. contg., by weight, 0.57-0.70% C, 0.15-0.50% Si, 0.70-1.50% Mn, 0.70-1.50% Cr and 0.03-0.50% Mo, contg. one or two kinds between 0.03-0.50% V and 0.03-0.50% Nb and consisting of the balance Fe with the elements as impurities. In said compsn., Mo forms carbide in the steel and fines austenitic crystal grains to improve the durability thereof and simultaneously has the working of preventing the hardening cracks thereof. The effect of ausforming can easily be obtd. and excellent hardenability can be obtd. as well by increasing the C amount.