Based on antenna's impedance and radiation performance, the slot loading effect on the TM-mode high-gain square patch antenna is investigated in this paper. Three different slot configurations are loaded to the patch along the central line, which can reduce the sidelobe in the E-plane. However, it is found that the H-plane beamwidth, impedance bandwidth and radiation Q factor become significantly different in these cases. At first, broader H-plane beamwidth, lower Q factor and wider impedance bandwidth can be obtained when only single slot is loaded in the center of the patch. In contrast, when two open slots are etched at two nonradiative edges of the patch, it will result in narrower H-plane beamwidth, higher Q factor and narrower impedance bandwidth. Moreover, better balanced performance can be achieved by simultaneously loading the central and sided slots. For validation, three kinds of antenna prototypes are designed, fabricated, and measured. The measured and simulated results agree well with each other, which demonstrate that different impedance and radiation performance can be freely adjusted and achieved by using different slot configurations.