This paper presents a Yagi-Uda antenna with a U-shaped dipole as the driver element for a terahertz photomixer. By constructing the antenna on a thin substrate, end-fire radiation patterns with high antenna impedance can be obtained even when the substrate has a high dielectric constant. The proposed antenna achieves a high input resistance of 4372 Omega at the resonant frequency by using a U-shaped dipole as the driver element. It is expected that such a Yagi-Uda antenna on a thin substrate will have a much higher terahertz output power than do conventional terahertz antennas.