In this communication, a single-layer differentially fed microstrip patch antenna (MPA) with wide bandwidth and low profile is proposed. The proposed antenna is composed of a patch radiator and a substrate integrated waveguide (SIW) cavity. In this work, a square SIW cavity is concentrically embedded within a square patch radiator. Here, the two dominant resonant modes in the radiative patch resonator and nonradiative SIW cavity, namely, TM10 and TE21, are both employed to achieve a wide impedance bandwidth under dual resonances. At first, the resonant properties of these two modes are investigated based on the cavity model theory. Then, their resonant frequencies are extensively discussed, aiming at reallocation of these two modes in proximity to each other by properly selecting the geometrical dimensions of the square patch resonator and SIW cavity. After that, the spacing between adjacent metallic pins of the SIW cavity is studied to reveal its influence on both radiation and bandwidth properties.