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A non-intrusive method for determining the time-resolved fuel mass flow rate in a hybrid rocket engine is proposed based on tunable diode laser absorption spectroscopy (TDLAS). The present work performed experimental tests using oxygen/paraffin as the propellant in a laboratory-scale hybrid rocket engine, applying oxidizer-to-fuel ratios in the range of 2.5–2.9 corresponding to combustion chamber pressures of 1.68–2.46 MPa. Variations in temperature and H 2 O partial pressure at the nozzle outlet were determined adopting TDLAS based on H 2 O absorption near 2.5 μm. Three H 2 O absorption lines at 4029.5, 4030.6 and 4030.7 cm−1 were simultaneously measured using only one diode laser at 2.0 kHz repetition rate and adopting a scanned-wavelength direct absorption strategy. The airflow velocity at the nozzle outlet was ascertained in two-dimensional numerical simulations. In this manner, the time-resolved fuel mass flow rate participating in combustion can be derived from the ideal state equation. A detailed discussion of the measurement uncertainty was provided herein considering the influence from determination of temperature, H 2 O partial pressure and velocity. The effectiveness of the novel method was validated by comparing its results with that acquired with the traditional weight-loss technique. Finally, it is initially analyzed that the combustion efficiency of pure paraffin fuel gradually increases during the combustion process according to the measurement results. • An optical fuel mass flow measurement method for hybrid rocket engines is proposed. • The quantitative fuel mass flow rate participating in combustion is obtained. • A compact tunable diode laser absorption spectroscopy system was designed. • Variations of temperature and H 2 O partial pressure at the nozzle exit were diagnosed. • Combustion characteristics of the oxygen/paraffin hybrid rocket were discussed. [ABSTRACT FROM AUTHOR]