The performances of planar SIW horn antennas will worsen significantly when the substrate thickness is much smaller than the free space wavelength λ 0 . Due to the mismatch between the horn aperture and the air, poor matching and undesired back radiation will be caused especially. In this paper, a novel thin SIW horn antenna, with a substrate thickness of 0.017λ 0 , loaded with a quasi-Yagi array is proposed to overcome these limitations in X band. To get a higher aperture efficiency, a metal-via-array lens have been placed inside the horn to correct the phase and amplitude distribution of electromagnetic (EM) wave on the horn aperture. Good agreements between the simulation and the measured results is obtained.