Abstract: The mechanical and fireretarding properties of medium density fiberboard (MDF) prepared by using phenol formaldehyde resin mixed with didecyl polyoxyethyl ammonium borate (DPAB) were compared with MDF control without DPAB. The results showed that there was no significant effect on modulus of rupture (MOR) nor modulus of elasticity (MOE) of the MDF when the dosage of DPAB added was below 1.2%. The oxygen index as well as the residual carbon content were increased with the increment of DPAB dosage, suggesting that the addition of DPAB could improve the fire retardation of MDF. Compared to MDF control, DPAB adding made the resin cure more completely according to the results of DSC analyses. However, the water uptake and swelling ratio of MDF upon water absorption were also increased with the DPAB loading due to the hydrophilic property of DPAB.