A single-layer microstrip patch antenna (MPA) under the operation of TM10 and TM20 modes for pattern diversity application without needing to rely on any feeding network is proposed for the first time in this letter. Initially, when a thorough analysis on mode excitation and inhibition of the MPA was deeply conducted, it demonstrated that its TM10 and TM20 modes could be independently excited by properly placing two coaxial ports underneath the radiating patch; thus, generating the desired broadside and bidirectional radiation patterns, respectively. Then, two sets of shorting pins are inserted below the core radiator, aiming to raise the resonant frequency of its TM10 mode in proximity to that of the TM20 mode. In addition, the maximum input resistances of these dual radiative modes decrease dramatically as f10 and f20 increase, which enables good impedance matching. Finally, the proposed MPA is designed, fabricated, and tested to validate the working principle. The results illustrate that this antenna can operate over the overlapping frequency ranges of 5.83-5.89 GHz with an envelope correlation coefficient of lower than 0.015, while keeping its low-profile thickness of 0.045λ0. Due to its unique advantages of low profile, simple structure, and good diversity, the proposed MPA could be a promising option in wireless local area network (WLAN)-related applications.