Abstract: Based on the weight losses of specimens caused by brown rot fungus (Lenntes trabea), the decay resistance of wood acetylated was measured. The results showed that all the control specimens which had not been treated belonged to the perishable class, and the treated specimens could be classified into the classes of moderately durable, durable or very durable.Microscopic and ultrastructure studies showed that in the treated specimens (average weight losses 0% -29.78%), no degradation appeared on the cell and cell-wall, hyphal existed only in the parenchyma and tracheid-lumen, and no penetration of hyphal through the wood cell wall was observed; while in the control specimens (average weight losses 73.80%-78.15%), both appearance of the cell and the cell-wall were degraded, with a large amount of hyphal, bore holes and the characteristics of late dacay.