In this paper, the slot loading effect on the TM mode high-gain square patch antenna is investigated in terms of its impedance and radiation performance. Three different slot configurations along the central line of the patch can all reduce sidelobes in the E-plane. However, the H-plane beamwidth, impedance bandwidth and radiation Q factor in these cases are significantly different. Initially, the central slot loading can help to achieve broader H-plane beamwidth, lower Q factor and the wider impedance bandwidth. In contrast, narrower H-plane beamwidth, higher Q factor and the narrower impedance bandwidth will be obtained by etching the slots at two edges. Consequently, more balanced performance can be obtained by loading both the central and sided slots.