This article presents a new linear calibration-free wavelength modulation spectroscopy (LCF-WMS) technique for gas sensing. By combining the natural logarithms of the transmitted intensity and background signals, this technique eliminates the effects of the intensity, intensity modulation, and the modulation phase to obtain integrable harmonic signals along the line-of-sight and thus realize calibration-free measurements. To illustrate the validity of the LCF-WMS technique, it was applied to measurement of the water vapor concentration in air at ambient temperature. Because of its specific features of line-of-sight integrability and baseline independence, this method will be advantageous in absorption tomography and high-pressure applications.