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In this paper, a highly-isolated microstrip patch antenna (MPA) is presented for pattern diversity applications. The physical insight of MPA is investigated without the influence of external exciter by using the theory of characteristic modes (TCMs). Shorting pins are added to manipulate the resonant frequencies of eigenmodes. TM₁₁, TM₀₁, and TM₀₂ modes are clearly tracked and identified. For the first time, all the three modes are simultaneously utilized to enhance the performances of MPA. To be specific, TM₀₁ and TM₀₂ modes have conical pattern, thus are combined to increase the bandwidth, while TM₁₁ mode has broadside pattern, and is used to provide pattern diversity. A two-port feed network, consisting of probe feed and aperture coupling, is designed to excite the three modes. The measured −10 dB bandwidth of the two ports is 720 MHz and 130 MHz. The simulated port isolation reaches 42 dB, and the measured value is higher than 27 dB. The proposed MPA has the advantages of clear operating principle, flexible mode tuning and high port isolation, thus is attractive in MIMO systems.