||In the manufacturing process of mechanical springs, high tensile residual stresses are generated which reduces considerably the spring strength and service life. These unfavorable residual stresses are partially eliminated by the heat treatment. In this process, the spring is heated uniformly below the material transformation temperature. An experimental investigation has been conducted to assess the stress relief influence on helical spring fatigue properties. First, S–N curves were determined for springs treated under different conditions (times and temperatures) on a testing machine specially designed to do this task. Next the stress relief effect on spring relaxation induced by cyclic loading was evaluated. Finally, residual stresses were measured on the inner and outer coil surfaces to analyze the effect of heat treatment. The most suitable heat treatment conditions (time range and temperature level) were obtained, these parameters give rise to the highest fatigue resistance with minimum spring relaxation.