Previous study indicates the PCDD/F concentration measured in stack gas of the Waelz plant investigated reaches 148 ng-TEQ/Nm3 due to its relatively low PCDD/F removal efficiency (<70%) achieved with baghouse filter (BF). In addition, the acidic substances such as SiO2 added into the Waelz plant investigated cannot reduce PCDD/F formation. In October 2005, Taiwan government set 9.0 ng-TEQ/Nm3 as the PCDD/F emission limits for existing Waelz plants and then 1.0 ng-TEQ/Nm3 was in effect starting from September 2006. The retrofit technology for reducing PCDD/F emissions from existing Waelz plant had been evaluated at the same time. Activated carbon injection (ACI) technology is adopted in the Waelz plant investigated in early 2006 to reduce PCDD/F emission. Flue gases samplings are conducted at different sampling points to evaluate the removal efficiency and the partitioning of PCDD/Fs between vapor/solid phases in the Waelz plant investigated. On the Step 1, the flue gas sampling results indicate that as the operating temperature of dust settling chamber (DSC) was increased from 480°C to 580° C, the PCDD/F concentration measured at DSC outlet decreased from 1,220 ng-TEQ/Nm3 to 394 ng-TEQ/Nm3. By applying ACI, the PCDD/F concentrations of stack gas decreased to 3.38 ng-TEQ/Nm3 at single baghouse filter (SBF) outlet as the activated carbon injection rate is controlled at 40 kg/h. On the Step 2, the ACI + dual baghouse filter (DBF) system is adopted in the Waelz plant investigated. The PCDD/F concentration measured at stack gas was further reduced to 0.203 ng-TEQ/Nm3. Hence, the ACI technology and DBF system adopted in the Waelz plant investigated can effectively reduce PCDD/F emission based on the flue gas sampling results.