A gear- box is the important continuously variable transmission in light truck. The structure and shape of a gear- box is particular complex and the phenomenon of stress concentration exists. And the high stress caused by stress concentration will directly affects the lifetime of a gear- box,even leads to crack. The structure strength of gear- box is studied through the experiment of static torque loading. The data of stress-strain characteristics of the cast aluminum shell is collected. And those data are analyzed with unidirectional stress method to find the stress concentration positions of a gear- box in its practical working conditions. Those analysis provide an important experimental basis for designing and manufacturing gearboxes.