To improve the detectable limit of radioactive aerosol monitors

it is very important to eliminate the inter-ference of the natural background

i.e. radon and thoron daughter aerosol. A particle size selective separator is used as artificial radioactive aerosol sampler. This separator may keep artificial aerosol from the natural radon and thoron daughter aerosol base on their different particle sizes. With the aid of the separator

most of natural background aero-sol can be rejected in sampling radioactive aerosols

which is very helpful to improving sensitivity of the radioactive aerosol monitors. The separation principle and calibration method of the separator are presented

test results are given

and some problems one must pay attention to are discussed in the paper.