A resonator feeding method is proposed in this paper to enhance the bandwidth and suppress the adjacent modes of a square patch antenna under operation of TM03 mode. The radiating patch is loaded with three slots for sidelobe reduction, and it is coupled to a half-wavelength resonator beneath, which is fed by a probe. On the one hand, the feeding resonator helps to generate an additional resonant mode, and thus widens the operating bandwidth. On the other hand, the feeding structure can effectively suppress the adjacent TM20, TM21, TM22 and TM23 modes, and this can contribute to more selective and stable gain performance. The simulated results show that a maximum gain of 14.3 dBi has been realized and the bandwidth is broadened from 1.8% to 3.6%.