Hydrothermal treatment of beech wood (Fagus orientalis) performed in acidic, neutral and alkaline buffered mediums in 160 and 180°C. White rot fungi (Coriolus versicolor) and brown rot fungi (Coniophora puteana) were used to determine the influence of heat treatment mediums on the behaviour of fungal resistance. White rot of wood digest lignin rather than cellulose; while brown rot digests the cellulose, but leave lignin behind. Results showed different behaviour of resistance against white and brown rot fungi. Heat treated specimens in acidic buffer medium showed the least weight loss in exposure to brown rot, but the weight loss caused by white rot fungi were even more than untreated samples. Heat treatment in neutral and alkaline buffer mediums caused less weight losses and nearly similar resistance behaviour to both fungi. Heat treatment of wood in acidic buffer medium lead to increase in crystallinity index and higher rate of lignin. Whereas the neutral and alkaline buffers controlled the carbohydrate degradation, considerably. Considering the decay mechanism of white and brown rot fungi, it can be concluded that specimen heat treated in acidic buffer had the highest rate of lignin that made it favorable for white rot fungi. But their low rate of carbohydrate made it resistant against brown rot fungi.