Sintering behavior and the contribution of soot produced in the absence of oxygen are experimentally investigated. A tandem differential mobility analyzer technique equipped with thermophoretic sampling was used to reveal the change in the mobility diameter of soot aggregates, which is proportional to the surface area. The mobility diameter decreased with an increase in reheating temperature. The decrease in mobility diameter clearly shows a decrease in the aggregate surface area. SEM images also clearly showed the decrease in the surface area and the simplification after heat treatment. These are experimental evidence that sintering of carbonaceous particle occurs in the absence of oxygen. Acetylene, which is considered to promote surface growth, is also added to size-selected soot particles before heat treatment to investigate the effect of surface growth on simplification of aggregate structure. The addition of acetylene, which is relevant to the surface growth via the hydrogen-abstraction/carbon-addition (HACA) mechanism, did not affect the mean mobility diameter at low temperature around 1200 K These results showed that simplification of soot aggregates would not be affected by surface growth caused via the HACA mechanism.