Abstract: Through the measure of escape ratio and varance of combustible gas on 93 species in Shaanxi at 240℃ the authors have showed that the relation between escape ratio and heated time can be described by model V=a+b/t,the variance of escape ratio during 10 minutes is the biggest among 6 different heated periods at 240℃ which means this index has an obvious effect on selection of fire resistance tree,and can be used as one of theoretical evaluation evidence for woody plant combustibility.According to gas release at 240℃,the fire resistance of Quercus variabilis,Lagerstroemia indica,Populus davidiana,Corylus heterophylla,Elaeagnus umbellata,Koeleuteria paniculata.,Picea wilsonii,Tsuga chinensis,Prunus armeniaca,Pyrus sp., Quercus glandulifera var. brevipetiolata is higher than the other mentioned species,and can be used as good fire prevention trees.