The transmission T and internal losses L of a non-planar ring oscillator (NPRO) laser are measured using a unique approach. Measurement of slope efficiency, axial mode separation, and relaxation oscillation frequency as a function of NPRO output power are all used in this procedure. T was assessed to be 2.3%, which was significantly higher than the supplier’s given value of 1.7% for a 32-deg angle of incidence at the NPRO output coupler at 1064 nm. The estimated value of internal loss was 4.2%.