Vertical diffusion in a stable boundary layer near the surface is not clearly understood. In the present study, the vertical diffusivity of radon and the height below which the concentration of radon is high are estimated from observations made at two levels (1.5 and 100 m) on the meteorological tower in the campus of the Meteorological Research Institute, Tsukuba, Japan, in November 2006. Seven 12-hour episodes, in which radon concentration near the surface increased, were averaged to make one sequential dataset. The averaged time sequence was divided into two periods: the radon concentration near the surface increased in the first one (Period I) and was stationary in the second (Period II). The estimated vertical diffusivity was less than 0.05 m2 s-1 for Period I and was at most 0.13 m2 s-1 for Period II. The estimated thickness of the radon-rich layer was less than 50 m for the first three hours in the first period; however, inversion height was approximately 100 m. The height under which radon accumulated was somewhat lower than that at the temperature inversion in period I, which suggests that the turbulent transfer was not dominant in the process to generate the temperature inversion layer in this area. Although the vertical diffusion of radon was different between the two periods, it was difficult to distinguish them with surface observation, employed for conventional Pasquill stability categories classification.