Abstract: Using varied weight of sample the dilution coefficient (φ=~(38)Ar/~(40)Ar)can be changed if the dilution dose of ~(38)Ar is known. In this paper variedweight of standard biotite ZBH-25 is used for measuring K-Ar age. Theresults show that the errors of ~(40)Ar determination bear no relation to dilutioncoefficient. This is consistent with the theory of error in isotope dilutionmass spectrometry. Therefore, the ~(40)Ar can be accurately measured so longas proper weight of sample weight of sample is chosen in the experiment.