The use of borate treated wood in outdoor is limited because of the migration of borate. The objective of this study is to investigate the fixation of boric acid in wood using polyethylene glycol (PEG-400) to form polyester borate and isocyanato group as coupling agent. FT-IR was used to monitor the reaction between boric acid and PEG. Elemental analysis was conducted to evaluate the C, H, and N variation in weight before and after treatment. Very low to undetectable level of boron was leached from the grafted sample suggesting a good fixation of boron to the wood compounds through 4,4'-Methylenediphenyl isocyanate (MDI) coupling agent. Laboratory soil block tests were conducted using white and brown rot fungi. At retention level of 0.79% BAE (Boric Acid Equivalent), an adequate protection against brown rot was obtained.