Abstract: In order to accurately determine the particle size distribution in mainstream cigarette smoke aerosol,a dilution system,which was composed of a diverter,an axial diluter and a 20 times jet-flow diluter,and the corresponding detection method for electrical low pressure impactor(ELPI) instrument were established.The experimental results indicated that:1) The dilution system met the requirements of ELPI for detection stability and instrument detection limits in the determination of mainstream cigarette smoke aerosol.2) The influence of test time on the test results was minor when detecting smoke aerosol with this ELPI-based method,and the inter-assay relative standard deviation(RSD) was less than 10%.3) This method featured a good stability,and RSDs of determination results of total mass numbers of aerosol were less than 15%.This method was found to be accurate,stable,reliable and suitable for determining the aerosol particles in smoke of cigarettes with different tar deliveries.