In recently, according to enlargement the application of steel bridges, we have a concerned with special properties of steel. It was necessary to develope the high strength, high weldability, high toughness. Thus, Thermo Mechanical Control Process(TMCP) steel developed as a structural steel. TMCP steel manufactured by controlled rolling followed by accelerated cooling process is known to have extra-ordinary mechanical properties such as tensile strength, weldability and toughness. In this study, the fatigue test was performed to evaluate the fatigue characteristic of TMCP steel which have softening at heat affected zone. To make it possible, the microcomputer connected with fatigue test machine was used, so it controlled the stress ratio automatically and calculated the relation of crack growth rate(da/dN) and stress intensity factor range(${Delta}K$) on base metal, heat affected zone and weld metal. From the result of the fatigue test, we can confirm that the influence of heat affected zone is not attentive on fatigue crack growth.