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A new graded screen array was designed for measuring the particle size distribution of unattached radon decay products. Its validity was verified by experimental approaches using a radon/aerosol chamber. Use of fine wire mesh screens achieved both a high volumetric air flow rate and high alpha count rate. Consequently, this improvement produced both a high sensitivity and good precision in the particle size distribution measurement for unattached radon decay products with an activity median diameter around 1 nm. A special Mylar film cover was used to protect the contamination of detector surface. And shielding effects by wire of a screen were examined in our alpha counting system for precise measurements. As for the results, the particle size distribution of unattached radon decay products was observed at around 1 nm as a narrow peak with the geometrical standard deviation of 1.1. When carrier aerosols were injected into the chamber, typical bimodal distribution was clearly observed. This clear bimodal distribution was observed even in an atmosphere with a relatively low radon concentration of 150 Bq m-3. It indicates that this system is adequate for measurements of the particle size distribution of unattached radon decay products in the natural environment. © 2004 American Institute of Physics.