The monitors used to measure radon concentration must be calibrated,and the calibration factor of each measurement cycle should be determined.Thus,the determination time of calibration factors of NRL-Ⅱ radon monitors should be reduced.In this study,a method is proposed to determine the calibration factors of radon monitors rapidly.In the proposed method,the calibration factor is initially determined in the 60-min measurement cycle;the calibration factor is then identified in the other measurement cycle on the basis of the principle that the calibration factor of the same radon monitor in different measurement cycles is inversely proportional to the number of a particles produced by ~(218)Po decay in this cycle.Results demonstrate that the calculated calibration factor of the different measurement cycles is consistent with the experimental calibration factor.Therefore,this method is reliable and can be used to determine the calibration factor of radon monitors rapidly. The monitors used to measure radon concentrations must be calibrated, and the calibration factor of each measurement cycle should be determined.Thus, the determination time of calibration factors of NRL-II radon monitors should be reduced.In this study, a method is proposed to determine the calibration factors of radon monitors rapidly. In the proposed method, the calibration factor is initially determined in the 60-min measurement cycle; the calibration factor is then identified in the other measurement cycle on the basis of the principle that the calibration factor of the same radon monitor in different measurement cycles is inversely proportional to the number of a particles produced by ~ (218) Po decay in this cycle. Results demonstrated that the calculated calibration factor of the different measurement cycles is consistent with the experimental calibration factor. Before this method is reliable and can be used to determine the calibration factor of radon monitors rapidly.