ABSTRACT Key words: PCDD/FS , aromatic precursor compounds , transition metal catalysts , chlorine donor , surface of fly ash particles In the last 20 years , increasing concern has focused on the environmental chemicals that mimic hormone functions , some of them toxic , which producing cancer , suppression of the immune system , and death from undefined causes . These chemicals are not made intentionally , but are formed as contaminants in combustion sources , including PCDD/FS of dioxin-like compounds that emitted from municipal solid waste incinerators (MWSI) and hazardous waste incinerators (HWI) . This study investigated the role that fly-ash plays in the formation of PCDD/FS using a commercial scale fluidized bed waste incinerators (FBWI) , which rated capacity at 3750000 kcal/hr (LHV) . In this design , a lay of sand is placed on the bottom of the combustion chamber. During combustion, the hot gases are channeled through the sand and crushed solid waste at relatively high velocity . This generated about eight times more mass of fly-ash will be produced from combustion zone than the others, and also makes much greater of PCDD/FS through the air pollutants control devices(APCD) to emission stack . The general reaction in this formation pathway is an interaction between an aromatic precursor compound and chlorine promoted by a transition metal catalyst on a reactive fly-ash surface. Since these reactions involve heterogenous chemistry , the rate of emissions is less depended on reactant concentration than conditions that promote formation such as temperature , retention time , transition metal catalysts (e , g,. Cu , Fe , Pb , Zn , Sn) and availability on catalytic surfaces of fly ash particle . These forming conditions will be proceeding a series of well study and experiment on fly-ashes from 4 zones (F1 , F2 ,F3 , F4) of FBWI . PCDD/FS synthesis from combustion of FBWI can potentially be explained by three principal mechanisms that results can be divided into several major parts as follows¡G 1. The fly-ash from zone F3 generated about 47 times more mass of PCDD/FS than zone F1 . 2. The F3 fly-ash proved to be the most active catalytic (Cu , Zn) medium , despite similarities with respect to specific surface area and average pore diameters . In addition , there are up to 75.9 percent by weight of Zn and 97.6 percent of Cu has been found in F3 of overall ashes. 3. In both fly- ash and transition metal catalysts in formation mechanisms are the dominant controlling factor for rates of PCDD/FS.