Laboratory wastes are discharged from experimental, testing, or analysis processes, and contain various toxic chemical compounds with a high heating-value and a high chlorine content (> 9%). Elevated PCDD/F (polychlorinated dibenzo-p-dioxin and furan) emissions during start-up stages are caused by combustion of waste with high chlorine contents, incomplete combustion, and so called "memory effects". Even though the duration of cold start-up is short compared with the hours of continuous steady operation in a waste incinerator, its negative effects with regard to PCDD/F emissions on both human health and the environment cannot be neglected. A full-scale laboratory-waste incinerator which is operated for 10 days in each run and has 15 to 20 runs annually was investigated in this study. Eleven PCDD/F samples of stack flue gas were collected during the cold start-up periods (for 60.5 hrs). The gas temperature of the primary combustion chamber was above 850°C, and was maintained at between 850 and 900°C by injecting diesel fuel without waste feed. For first 1.5–7.5 hours, the PCDD/F concentration in the stack flue gas was as high as 0.656–1.15 ng I-TEQ/Nm3. Afterward, during hours 10.5–35.5 and 54.5–60.5, this reduced to 0.159–0.459 and 0.218–0.254 ng I-TEQ/Nm3, respectively. Based on principal component analysis (PCA) and the L/H ratio, the results revealed a higher L/H ratio (1.23) before hour 32, indicating that less chlorinated PCDD/F homologues (tetra and penta) dominated, while after hour 32 more chlorinated PCDD/F homologues (hexa, hepta and Octa) had a higher mass concentration and the L/H ratio fell to 0.42. These results indicate that the PCDD/F emissions during cold start-up were caused by memory effects and thermal desorption. Therefore, in order to reduce the PCDD/F emissions from the stack flue gas of waste incinerators, is highly recommended that a higher amount of activated carbon injection is used in front of the bag filters.