Reservoir structures are often constructed in the interconnection to prevent the electromigration damages. In this study, a numerical simulation technique for analyzing the atomic density distributions in the line under high current density was used to evaluate the effects of reservoir length and location on the threshold current density considering void and hillock generations. The threshold current density is determined when the local atomic density in the line reaches the upper critical value for hillock creation or the lower critical value for void generation. Atomic density distributions in the line were simulated when cathode and anode reservoir lengths were changed. The threshold current density considering void formation became higher with longer cathode reservoir and shorter anode reservoir. However, opposite results obtained in the case of hillock formation. It was found that there was an optimum value of reservoir length, corresponding to both critical values of hillock and void initiation.