Accelerated stress-migration testing under 200℃ of Cu (M1/M2) interconnects has been done for 700h. Finite element analysis and focused-ion beam techniques have been used to study the stress-induced voiding in the Cu interconnects with vias of 500 and 350nm in diameter. The voiding mechanism and the effect of via size on the stress migration have been studied. The results show that peak values of stress and stress gradient in M1 lines are reached underneath the edge of vias. The stress gradient shows crucial effect on the voiding process. The vacancies introduced by thermo-mechanical stress diffuse along Cu M1/SiN interfaces under the force of stress gradient and nucleate at the peak values of the stress gradient. The void grows faster along the length direction because the stress in M1 lines changes faster horizontally. The stress and stress gradient increase with increasing via diameter, leading to a faster voiding rate.