In this paper, we proposed a novel design of U-slotted SIW antenna. Our antenna design is aimed to cover upper K-band and lower Ka-band spectrums, specifically from 24 GHz to 32 GHz. It has a compact square size of 5.2 x 5.2 mm2. We use a rectangular truncated corner to optimize the square radiator. The optimized rectangular truncated corner size of 2 x 0.8 mm2 gives an impedance bandwidth of 7.87 GHz. SIW cavity is constructed by using multiple metallic via-holes which are drilled in a dielectric substrate establishing. Next optimization, applying the U-shaped slot and SIW structure yield a wider impedance bandwidth of 8.89 GHz, there is about 1.02 GHz of impedance bandwidth enhancement. In addition, the SIW structure gives a higher gain of 7.63 dB and decreases the sidelobe level of -12.1 dB. Implementation of the SIW structure significantly decreases the size of antenna while keeping the antenna parameter’s performances.