The fatigue and tensile test were carried out with Mild steel using the Dump Frame of commercial car. The specimens were heat-treated at $810^{circ}C$ and $930^{circ}C$ and worked 4.5, 6.0, 8.0mm thickness in order to look over the mechanical properties and fatigue life by heat treatment and thickness from the tensile test result, the yield strength of the heat treated specimens was increased about 35% more than that of the non-heat treated specimen. The fatigue life of non-heated specimen was decreased 15% but that of heat treated specimens at $870^{circ}C$ and $930^{circ}C$ were decreased 16.38% and 13.16% respectably according to increasing the thickness from 4.5 to 8.0mm.