Behaviors of dioxins in remediation of dioxin-contaminated soil by thermal desorption and relationship between the behavior and the vapor pressures were investigated. PCB-contaminated soil prepared for this study and actual contaminated soil sampled at a chemical plant were used. The performance of thermal desorption was better at higher temperature, and it changed significantly between 300 degrees C and 400 degrees C. A part of dioxins was decomposed in thermal desorption by catalytic action of soil. The remaining fraction correlated with the vapor pressure of each congener, and the correlation was caused by the length of reaction time with soil.