Stable-isotope ratio analysis of water is an important tool for geology, meteorology, and earth sciences. Measurements of water vapor isotopes are very helpful to explaining stratospheric aridity and related issues in atmospheric sciences. The absorption of water vapor near 2.7μm is very strong so it is suitable for measuring high sensitivity spectra. Based on difference frequency generation and quasiphase matching, by mixing an Nd∶YAG laser with Ti∶Sapphire tunable from 750 to 840 nm in a 50 mm long periodically poled lithium niobate(PPLN) crystal, a widely tunable CW laser source was generated for the midinfrared spectral range from 2.5 to 4μm. We chose A=20μm for PPLN crystal, the generated laser was around 2.7μm. This laser is widely tunable and of inherent narrow linewidth based on difference frequency generation. Using this idler laser and 100m multipasscell, and direct absorption the water vapor isotopes were measured in the laboratory air. The authors measured isotopes ratios and δ¹⁷O,δ¹⁸O and δ D. The values were found to be in excellent agreement with thestand ardvalue for three individual lines.