Microscopic evaluation of microbial attack in the acetylated wood after a long term exposure to the field soil was the main concern of the current research. Stakes of wood were cut from beech, poplar and Scots pine wood and acetylated by using acetic anhydride to achieve different weight percent gains (WPGs). Afterwards, the stakes were exposed to the field soil for 350 weeks according to EN 252. Small specimens were cut from the stakes, sectioned and stained for microscopic studies. Small blocks were also prepared for scanning electron microscopy (SEM).Microscopic observations indicated that the soft rot was dominant rot fungi in the field soil and decayed the non-acetylated wood severely and attack also the moderately acetylated wood slowly. Bacteria were also in close association with the soft rot fungi to degrade the acetylated and the non-acetylated wood. In the moderately acetylated wood, white rot decay was at developing stage, while it was at initial stage at the highest WPGs.Microscopy revealed that the initial stage of the decay was begun at the highest WPGs and it was at developing stage of the decay in the moderately acetylated wood. The less development of the decay in the acetylated wood comparing the non-acetylated wood proved very good protection of the wood against the soil microorganisms due to the acetylation after 7 years period.Soft rot and white rot decays were observed in the non-acetylated and the acetylated wood at the same time. No brown rot decay was observed in those woods. Bacteria were seen in all wood types. The bacteria cooperated generally with the soft rot fungi to degrade the non-acetylated wood or limit themselves to pits at the higher degrees of the acetylation.