The ULSI is a composite system made from many materials and the stress is generated caused by the mismatch of the thermal expansion. The reliability issue related with the thermal stress is mainly discussed about the thermal history of the fabrication processes. However, the global bus line is facing a new mode of failure during operation because of the increased thermal power at high speed operation and the reduced strength of low-k dielectric material. It has been pointed out that attention should be paid to thermal stress in designing the floor plan of the global bus lines.