This paper discusses the differences between oxygen index ( OI ) of wood on vertical and transverse section by four wood species with five fire retardants. The results showed that the OI of vertical section was lower than that in transverse section because wood is anisotropy and coefficient of heat conduction is different in different direction .But there was no obvious difference on radial and tangential section since the radial section is similar to tangential in structure. The effect of wood species on OI of wood was different, high density wood had a greater OI . With fire retardant treatment, the OI of vertical section could be higher than transverse section. The effect of same retardant on different wood species was different.