The radon self-migration law is discussed. It is shown that

in the air under an idealized condition (no convection

no pressure

temperature and potential differences

and sealed)

the contribution of diffusing of radon and its daughters to their migration is less than 10%

the descending purely caused by gravity is less than 45%; and the contribution purely given by buoyancy is more than 45%. The radon measuring method and application in geology is also introduced in the paper.