Radiation efficiency is one of the key parameters for smartphone antennas. However, the actual mechanism governing the radiation efficiency remains unclear, and few works have been published. This work is an attempt to extend theory of characteristic modes (TCM) to the lossy smartphone antenna for gaining insight into what causes efficiency variation and realizing efficiency improvement. Compared with the classical TCM, the characteristic mode analysis (CMA) adopted in this work has its capacity to handle lossy smartphone antenna. By using such analysis approach, we find, quite interestingly, that suppressing the higher-order modes is an effective optimization strategy to enhance efficiency. Then, an optimized antenna with considerable improvement in efficiency is proposed. Finally, to meet the requirement of practical application, the additional LC circuit and through-holes are introduced into this proposed optimized antenna. GSM900 of 890-960MHz can be covered by this finally proposed antenna, and the measured peak total efficiency is improved from 26% to 43%. Due to the attractive features of small heights (4mm) and tiny clearances (0.25mm), the proposed designs are promising for application in smartphone with full-view display and thin profile.