A stripe waveguide was fabricated by organic-inorganic hybrid sol-gel technology. The waveguide was connected into a Sagnac fiber loop to measure output power under environment of different ethanol vapor volume fraction. It was found that the output signal varied in a sinusoidal function of ethanol vapor volume fraction within the measured range. According to the property of Sagnac loop, the measured behavior indicates that there exists birefringence caused by ethanol vapor absorption into the sol-gel material. The birefringence phase shift was observed to vary with ethanol vapor volume fraction in a sub-linear way. The coefficients of the linear term and second order term were calculated from the experimental data fitting, and the birefringence sensitivity to ethanol volume fraction was evaluated to be Δn≈4.4×10-2. The temporal signal change was also measured, and the typical rising and falling time constants were obtained to be 3 min and 12 min respectively.