Disclosed herein is a new method for continuous heat treatment to be applied to the production of oil tempered steel wires for springs having high strength and high toughness to meet the requirement for weight reduction. The heat treatments are applicable to a medium carbon low alloy spring steel which does not undergo martensitic transformation substantially upon oil hardening alone. It comprises performing two-step accelerated hardening consistin of oil hardening and immediately following water hardening and subsequently performing tempering. The medium carbon low alloy steel is one which consists 0.40-0.65% carbon and Si and Mn as essential components and further at least one species of Cr, Ni, Mo, and V, and have the chemical composition corresponding to and Mf point lower than 80.degree. C. (preferably 10.degree.-70.degree. C.). It is desirable that the oil be wiped from the steel wire after the oil hardening and before the water hardening.