Abstract: In order to determine the influences of forest fire on the flashover property and mechanism of air gap under high voltage transmission lines,the flashover characteristics of simulative AC high-voltage transmission lines（single-bundle conductor,double-bundle conductor and four-bundle conductor） under the effect of wood crib fire were investigated and compared with the gap breakdown characteristics in air.Results show the average breakdown field strengths of transmission lines are remarkably decreased by wood crib flame.The measured ratio values of the average breakdown field strengths of gap in stable flame and in extinguishing flame to those in air（27.3%～33.5% and 39.4%～48.4%,respectively） are found to be lower than the calculated ratio values（37.4% and 49.8%,respectively）,which are obtained by the proposed formulas that calculate the breakdown field strength under the high temperature field of wood crib fire.These results indicate that high flame temperature is the primary factor while flame ionization is secondary contributing to the reduction of breakdown filed strength.