A liquid scintillation spectrometer has been applied to the determination of the counting efficiency for the air luminescence produced by standardized Rn-222 and its daughters. Based on the counting efficiency, known amounts of Rn-222 can be prepared in 3.5 h. Moreover, the Ra-226 source from which Rn-222 is collected does not necessary have to be a standard one. In contrast, the conventional method for preparing known amounts of Rn-222 usually requires a long waiting period(ca. 15-30 days).