The PCDD/F emissions from incinerator start-up are a major contributor to the total amount of such emissions, as has been reported in studies of numerous municipal solid waste incinerators. However, very few studies have examined the start-up process at hazardous waste incinerators (HWIs). In this work we studied the emissions of PCDD/Fs and other pollutants, such as PCBz, at the stack during start-up and normal operations in a HWI. We found that the PCDD/F emissions during start-up were greater than during normal operations, and were comparable to the annual emissions during the normal combustion regime. The emissions of PCDD/Fs were highest during start-up when the temperature was around 500°C, reaching 59.5 ng/Nm3 (5.49 ng I-TEQ/Nm3) when no APCDs were applied. The emission values of PCDD/Fs during start-up with the application of APCDs and during normal operations were very low, which indicates the importance of APCDs when starting up a HWI, as well as before feeding waste. The chlorination degree, the ratio of PCDFs/PCDDs and the congener profiles were also discussed during start-up and normal operations, with the results suggesting different formation mechanisms of PCDD/Fs. PCBz emissions are two or three orders of magnitude higher than those of PCDD/Fs, and 1,2,4,5-TeCBz was the best correlated PCBz used as a PCDD/F indicator in real HWI flue gas.