Experimental study was conducted to clarify the formation behavior of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from carbonaceous materials through a de novo synthesis route. Samples were prepared by changing mixing method and composite state of metallic chloride in graphite powder in order to simulate the texture of “unburned carbonaceous particles”, i.e., soot, formed in thermal processes. Reagents of KCl, CaCl2·2H2O, FeCl3·6H2O, and CuCl2·2H2O were used as chlorine sources and were added to graphite powder with different methods. The composite state of metallic chloride was varied by preliminary treatments:  hand-mixing, mixed-grinding using a high-intensity ball mill, and preheating at different temperatures between 500 and 1100 °C. In the de novo experiment, reaction temperature and oxygen concentration of flowed gas were set at 300 °C and 2.5 mol %, respectively. During the experiment, formation rates of CO and CO2 and the formed amounts of organic chlorine and PCDD/Fs were measured. The results show a reasonable relation between the amount of formed organic chlorine and oxidation rate of carbon, and the order of the activity of metallic chlorines was obtained as KCl < CaCl2 < FeCl3 ≪ CuCl2. Furthermore, it was found that the effect of the composite state of metallic chloride on the formation of PCDD/Fs significantly depends on the kind of metal. The results will give useful information to examine the formation mechanism of PCDD/Fs from unburned carbon particles in thermal processes.