BGA type LSI package is composed by different materials in thermal mechanical properties. Therefore, destruction of a solder ball may arise with various thermal loads in a manufacture process and the environment. In this report, the thermal stress was analyzed by Finite Element Method from the viewpoint from which the optimization design indicator of BGA type LSI package was requested. Concretely, the thickness of the adhesive film included in the package was changed, and the amount of the maximum warp deformation, the maximum principal stress and the maximum shearing stress in the solder ball was examined in each cooling process of assuming the manufacturing process. It was clarified that the adhesive film thickness from 0.3 mm to 0.4 mm was preferable from the point of the maximum warp deformation, the maximum shearing stress and the maximum principal stress in the range of this examination.