We propose a ground decomposition method (GDM) to improve the performance of mobile antennas. In the GDM, overall ground impedance is analyzed as the combination of independent decomposed ground parts. In a proposed design, the overall ground mode resonance is tuned by adjusting the impedance of a flexible printed circuit board, which is decomposed according to GDM. This greatly improves the radiation performance, as verified through simulation and measurement. This proposed method is effective for mobile antenna design, and especially for actual devices that have very complicated ground configurations.