It was confirmed by the existing studies that, when it was exposed by only radiant heat, leaves of many species of trees ignited without flame but Juniperus chinensis cv. Kaizuka flamed lower than the tolerance limit of radiant heat. (It is said the lowest heat flux level of glowing as to coniferous trees was 13.95kW/m².) The research on such danger is not progressing but which should be verified, for utilizing the fire prevention function of trees and for planning landscape in narrow residence, such as city. So, in this experiments, the concentration and transition of unburned combustible gas and the temperature under the leaves were measured about 23 species of trees, in this time the hydrocarbon was mainly measured as the unburned gas. The following results were obtained: (1) The states of hydrocarbon generating and temperature transition were obviously different depending on the species. (2) Three species of the family Cupressaceae discharged much hydrocarbon from the first stage of heating as well as J. chinensis cv. Kaizuka. (3) Aucuba japonica, Prunus mume, Phyllostachys bambusoides, and Pleioblastus chino generated hydrocarbon few from beginning to end. (4) Viburnum awabuki, Ginkgo biloba, and Hydrangea macrophylla generated few in first stage compared with the total quantity.