A dual-band single-layer microstrip patch antenna (MPA) with simultaneous improvement in impedance bandwidth and radiation beamwidth is proposed in this communication. The performances of the MPA are implemented by resonating its radiative TM10 and TM12 modes at the lower and upper bands, respectively. Initially, the far-zone radiated fields of the MPA working in the TM10 mode are theoretically investigated. It demonstrates that its E-plane half-power beamwidth (HPBW) in the lower band is significantly widened by removing the array factor between dual equivalent slots. After that, the MPA working in TM12 mode is deeply studied and analyzed. The results depict that its wide E-plane HPBW and low H-plane sidelobe level (SLL) in the upper band could be obtained by reshaping radiation patterns of its improved TM12 mode via the shorting pins. Additionally, a linear slot is cut on the core radiator so as to achieve a good impedance matching. With these arrangements, the E-plane HPBW, H-plane SLL, and impedance bandwidth of this designed MPA can be simultaneously enhanced in the dual bands. Finally, the proposed antenna is fabricated and measured. The experimental results show that the antenna has gained an improved bandwidth (|S11| < –10 dB) ranging from 3.45 to 3.77 GHz and 5.75 to 6.04 GHz with two attenuation poles. Most importantly, the HPBW of its E-plane radiation pattern is successfully widened to around 135◦ over these dual operating bands. Moreover, the high H-plane SLL of the antenna is decreased to about –10 dB in the upper band.