Abstract: This paper studied resistant mechanisms of slash pine to brown spot needle disease by comparing with the needle tissue structures and resin contents. Results indicated that densities, opening diameters and structures of stomata, as well as sizes, arrangements and densities of mesophyll cells had no obvious difference between resistant and susceptible clones. But, mesophyll levels, numbers of resin duct and resin contents of resistant clones were evidently thicker, more and higher than those of susceptible clones. There were no differences in the features of bundle endodermis and transfusion tissue. Histopathological analysis showed that the hyphae developed only in mesophyll tissues and did not get into endodermis. The hyphae distributed mainly near the propagation bodies, but, in the susceptible clones the hyphae may be seen also at far from propagation bodies.