We present a lab-on-chip technique to measure the very low losses in superconducting transmission lines at (sub-) mm wavelengths. The chips consist of a 100 nm-thick NbTiN Co-planar Waveguide (CPW) Fabry–Pérot (FP) resonator, coupled, on one side, to an antenna and, on the other side, to a Microwave Kinetic Inductance detector. Using a single frequency radiation source allows us to measure the frequency response of the FP around 350 GHz and deduce its losses. We show that the loss is dominated by radiation loss inside the CPW line that forms the FP and that it decreases with the decreasing linewidth and increasing kinetic inductance as expected. The results can be quantitatively understood using SONNET simulations. The lowest loss is observed for a CPW with a total width of 6 μm and corresponds to a Q-factor of ≈15 000.