The effect of valid particle diameter of activated carbon on dioxins removal efficiency was explicated using a fixed bed column and 1,2,3,4-tetrachlorinated dibenzo-p-dioxin (T4CDD). In the adsorption process of T4CDD on to activated carbon, the mass transfer rate of T4CDD in a boundary film on activated carbon could be evaluated by the valid diameter calculated from the pressure drop through the packed bed. The mass transfer rate of T4CDD in the pores of activated carbon increased with increasing roughness of the surface of activated carbon particles. The dioxins removal efficiencies of a bag filter with activated carbon injected into the inlet flue gas and an adsorption tower filled with activated carbon were estimated quantitatively, using the mass transfer model and the linear-isotherm system (Henry's law). The adsorption coefficient of activated carbon with a surface area of 181 m(2)/g for pores of 2-50 nm in diameter was estimated to be 6.1 x 10(5) m(3)/kg at 170degreesC.