A comprehensive study on the mechanism of bandwidth enhancement for an equilateral triangular microstrip patch antenna (MPA) under differential excitation is presented in this paper. Attributing to the way of differential excitation, the radiation directivity of TM 11 mode in such a patch has been improved a lot, and field distributions for both TM 10 and TM 11 modes are found to be simultaneously excited for radiation. Next, two shorting posts are introduced to this MPA so as to generate an additional mode, i.e., zero mode, which resonates in a frequency between TM 10 and TM 11 modes. With the proper arrangement of these three modes in proximity to each other, a wide bandwidth for radiation can be satisfactorily achieved. To demonstrate the validity of the proposed concept, a prototype antenna is finally designed and fabricated. As theoretically expected, the impedance bandwidth of the final designed patch antenna has been increased to 50.46% and the gain is around 6.5 dBi. Both simulated and measured results exhibit wide bandwidth and good performance of radiation.