The purpose of this study was to produce urea-formaldehyde based composite particleboard from walnut shell and improve its requirements such as non-flammability, eradication of fungal and insect attack, and water resistance features by using fly ash and phenol-formaldehyde. For this purpose, under laboratory conditions, parameters affecting polymer composite particleboard from walnut shell and urea-formaldehyde were specified to be urea-formaldehyde ratio, reaction temperature, and reaction period, and the effect 2 of these parameters on hardness was investigated. The optimization results showed that maximum bending strength was 3.8 N/mm(2,) at urea-formaldehyde ratio of 1.0, reaction temperature of 70 degrees C, reaction time of 25 min, walnut shell filler/urea-formaldehyde resin of 3 and mean particle size of 0.12 mm. As a result of burning tests, although the maximum flame temperature of composite particle board without fly ash was 535 degrees C, it was 299 degrees C with fly ash of 15% (w/w) according to the filler. Water absorption and increasing in the thickness exponentially decreased with increasing phenol-formaldehyde/urea-formaldehyde ratio. (C) 2006 Elsevier Ltd. All rights reserved.