A mid-infrared two-line thermometry based on H2O absorption was developed for in situ and sensitive temperature measurements of laminar flames. The H2O absorption lines in v(3) fundamental band were systematically investigated and a new promising line pair (4029.52 cm(-1) and 4030.73 cm(-1)) was selected to achieve strong absorption line-strength and high-temperature sensitivity over the temperature range of 1000-3000 K. This line pair was readily assessed by a single current scan of a distributed feedback laser near 2.5 m. The scanned-wavelength laser absorption strategy was adopted for temperature measurements for both laminar sooting and non-sooting flames. This optical sensor was applied in measuring various flame conditions including CH4/air and C2H4/air premixed flames (? = 0.8-1.2), C2H4/air sooting flames (? = 1.78-2.38), and partially premixed flames. Our measurements were in good agreement with thermocouple measurements for all the flame conditions. In addition, numerical simulations using the detailed chemical kinetic mechanisms (CH4 flame: GRI 3.0, C2H4 flame: USC 2.0) were conducted to validate the experimental results.