Stratospheric air samples were collected over Japan on August 31, 1994 using a balloon-borne cryogenic sampler and then analyzed for the CH4 concentration and δ13C in CH4. The δ13C value increased from about −47.5‰ at the tropopause to about −38.9‰ at 34.7 km, accompanied by a rapid decrease of the CH4 concentration with height. From the measured values of the CH4 concentration and δ13C, the apparent carbon isotopic fractionation factor was estimated to be 1.0131±0.0006. The contributions of the chemical CH4 reactions with OH, O(¹D) and Cl were also examined by using an one-dimensional photochemical-diffusion model. It is suggested from this examination that the reaction of CH4 with Cl is especially important for δ13C in the stratosphere.