In this Letter, a novel compact slot antenna has been realised using substrate integrated coaxial line (SICL) cavity. SICL technology helps to achieve cavity size of while maintaining its inherent advantage of self shielded and low loss transmission line at millimetre frequencies. A SICL cavity is designed by shorting the middle conductor at the two edges and keeping the other two edges separated from the sidewall. The eigenmode analysis affirms the presence of a dominant TEM mode at 28 GHz along with the presence of SICL even and odd modes. A half wavelength diagonal slot is etched on the bottom plate of the cavity to disturb the TEM mode surface current at 28 GHz. Measurement results show the SICL cavity backed slot antenna exhibits a gain of 5.6 dBi at 28.6 GHz and high front-to-back ratio better than 17 dB. The SICL-based cavity exhibits millimetre wave resonance with much smaller size, which helps to improve the compactness of the design. The proposed antenna can be used for millimetre wave applications like handheld mobile devices at millimetre-wave frequencies.