The flammability limits of methane have been reinvestigated by using the AC discharge ignition method. This work has partly been motivated by the necessity of evaluating the flammability characteristics of CFC alternatives. In the study, the effects of spark duration time and spark gap on the flammability limits of methane have extensively been explored. The resulting flammable range tends to become wide if the spark duration time is too long and/or the spark gap is too large. The effects of inadequate spark duration time and spark gap are exaggerated if the height of the experimental vessel is too small. As a result, the spark duration time of 0·1–0·2 sec combined with the spark gap of 6–8  mm has been found to make an optimum condition for the correct measurement of the flammability limits by the AC discharge ignition method.