A novel approach is developed for measuring small losses in highly transparent Si3N4/SiO 2 waveguides on a silicon chip. The approach is particularly applicable to waveguides written by high-resolution patterning techniques, such as e-beam lithography, whose lengths cannot be easily increased beyond several centimeters. This method is based on measuring the transmission of an optical cavity formed by two highly reflective (R at least 0.999) simple Bragg gratings and a uniform waveguide between the two gratings whose length can be varied to increase the loss fitting accuracy. A theoretical model based on an ABCD matrix method is developed and used for the final loss value fitting. Experimentally, a cavity with extinction ratio over -70 dB and quality factor Q = 1.02×10 ⁶ is realized. The fitting results show a waveguide loss of 0.24 ± 0.01 dB/cm and a grating loss of 0.31 ± 0.01 dB/cm. These results are obtained with relatively high index contrast (Δn > 0.001) gratings with 0.1-pm wavelength scanning resolution. It is expected that with better design and wavelength scanning technique, this approach is applicable more generally to measure waveguide loss coefficients as low as 0.001 dB/cm.