Abstract: Radon and its progeny contribute half of all radiation exposure of the public received from natural radiation, while the unattached fraction of radon progeny plays an important role on its dose conversion factor. To understand better on the variation of the unattached fraction of radon progeny and how environmental factors work on it, both 24 h integrated monitor and real-time continuous monitor were adopted by this study for the observation on annual variation and diurnal variation as well. The results of monthly field measurement show that the annual average of the unattached fraction of radon progeny is 8.8% (6.4%-12.5%) and 9.7% (4.9%-12.8%) for indoor and outdoor, respectively. In indoor environments, the unattached fraction of radon progeny is more stable compared with that of outdoors since indoor environmental factors are more stable. For diurnal variation, even though radon equilibrium equivalent concentration has a disciplinarian variation, higher in the morning and lower in the afternoon, the variation of the unattached fraction of radon progeny does not show any regularity.