A patch antenna with loading of slot and shorting pins is proposed in this paper to enhance the radiation gain without increasing the patch size. At first, a straight slot is transversely introduced in the central line of a patch. For the dominant TM01 mode, this central line behaves as a virtual electric wall with zero electric field and maximum surface current underneath and on the patch, respectively. By doing so, the central slot serves as an extra radiator so that this slot-loaded patch produces radiation as an equivalent three-slot array. Meanwhile, a pair of shorting pins is longitudinally placed at the two symmetrical sides of this central slot to maintain the resonant frequency. Due to the out of phase between the electric fields in the central slot and two sided slots, the far-field radiated by the three slots can be canceled with each other in specific directions away from broadside. Thus, this cancellation results in sidelobe of low level in the E-plane due to emergence of radiation nulls, and further makes the main radiation beam more directionally selective. Accordingly, the directivity and gain of this proposed patch antenna are enhanced with no need of enlarging the overall size of patch. Finally, the simulated and measured results are provided to validate the operating principle and show that the directivity is increased by about 1.7 dB increment.