Based on the finite element analysis and the automatic modeling, the interconnect reliability for GaN monolithic microwave integrated circuit (MMIC) power amplifier (PA) with different transistor sizes, currents, and temperatures have been predicted effectively in this article. The modeling and computation are completed automatically based on the ANSYS Parametric Design Language (APDL) to study the electromigration (EM) failure of interconnect. By controlling the temperature, the current and the transistor size, the atomic flux divergence (AFD) of interconnect in different states for this PA has been obtained. It can be concluded that the increase of the temperature and the current, and the decrease of the transistor size may promote the EM failure. Furthermore, the sensitivity analysis for AFD of this model is performed too. Then, a feasible solution space can be obtained which can help the designer to get the reliability performance for any design requirement and the trade off decisions on the transistor size and the operation condition. This is the first study of interconnect reliability for a high power GaN MMIC PA, thus the interconnect reliability research for complex IC can be further explored.