A method has been developed for the simultaneous measurement of radon ( 222 Rn) and thoron( 220 Rn) in a mixed environment using a scintillation cell in this paper. The method uses counts from two arbitrary counting intervals from zero time with respect to sampling,establishes two equations with two unknown variables to solve concentrations of radon and thoron,hence,called two-count method( TCM) . The method can be used to esti-mate the radon and thoron concentrations in a mixed environment accurately and quickly, as there is no nee...