The aim of this study was to determine the effect of tannin content of urea formaldehyde (UF) resin on the panel properties of medium density fiberboard (MDF). Tannin extracted from the bark of white oak (Quercus alba) was added to UF resin at different ratios (based on the resin) to decrease the free formaldehyde content of MDF panels in this study. It was determined that free formaldehyde values of MDF panels decreased when the ratio of tannin content in UF resin increased. However, the modulus of rupture (MOR), modulus of elasticity (MOE), and internal bond strength (IB) of these panels were a little lower, and the thickness swelling (TS) (24 h) and water absorption (WA) (24 h) values were a little higher compared to the control MDF panels. (C) 2011 Elsevier Ltd. All rights reserved.