The OH radical concentration was measured by applying tunable diode laser absorption spectroscopy, which is an in situ optical method. An optical absorption region (P7.5ff transition at 1502.7 nm) of the OH radical was selected in the near-infrared range to measure the OH radicals quantitatively in premixed CH4/air flames. An improved direct absorption spectroscopy (DAS) method based on wavelength division multiplexing was proposed to extract the H2O absorption signal that interfered with the OH light absorption signal, and the integral intensity of OH* chemiluminescence was compared to the measured OH radical concentration based on the improved DAS method.