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Radon is the most important source of ionizing radiation which poses human health risks. There are three naturally occurring radon isotopes which are Rn-222(radon), Rn-220(thoron) and Rn-219(actinon) with respect half-life of 3.8days, 56 seconds, and 4 seconds. They are formed on the alpha decay of their radium parents Ra-226, Ra-224 and Ra-223 which are members of decay series of U-238, Th-232 and U-235, respectively. The scientists and researchers are continuing designing and developing methods and techniques for measurements of Radon. We are developing a method which can be used to measure the activity concentrations of all three naturally occurring radon isotopes. To measure radon activity concentration we choose the isotope having short half-life from the decay chain. For example to measure Rn-222 activity we select out Po-214 in U-238 decay chain and we set the time difference from 2 μs to 656 μs. The lower limit of 2 μs is adopted to reject the short Bi-212→Po-212 coincidence in the Th-232 chain. This 2 μs is near about 7 half-lives of Po-212. The upper limit was set as 656 μs which is about four half-lives of Po-214. We will report on development of this method in which the analysis of the method will be presented and discussed.