A fixed-tuned D-band third-harmonic mixer is presented with broad bandwidth and low conversion loss (CL) based on a compact transition-match structure (CTMS). The CTMS is composed of a short suspended stripline (SSLIN) on the top side and a triangle-shaped transition on the bottom side, located in the radio frequency (RF) waveguide. The mixing diode with two Schottky junctions in series is located directly on the short SSLIN in the RF waveguide, designed to be at the maximum of the electrical field of the RF signal. The single balanced configuration is then achieved, and the RF signal is only mixed with the odd harmonics of the local oscillator (LO) signal. The triangle-shaped transition helps to minimize the discontinuity between the waveguide and the microstrip circuits. The mixing circuit is miniaturized and the mixer bandwidth is broadened benefiting from the CTMS. Dimension of the circuit substrate is 5.1 mm×6.9 mm×0.127 mm. The measured results indicate that the single-side band (SSB) CL of the mixer is typically 13.0 dB, and ranges from 10.8 dB to 15.7 dB over 135-165 GHz. The instantaneous intermediate frequency (IF) is available up to 15 GHz. Keywords: terahertz (THz); wideband; third-harmonic mixer