Objective To investigate the concentrations of indoor radon (222Rn) and its daughter products as well as indoor thoron (220Rn) in selected houses in Yuhang district and Sanmen county,Zhejiang province,and estimate their annual effective doses to the population.Methods Solid state nuclear track detectors were used in selected dwellings in Yuhang district and Sanmen county,and the detectors were placed in bedrooms or living rooms.Without changing the ventilation habits of residents,These detectors were continuously placed from March to September in 2009.Results Indoor 222 Rn and 220Rn concentrations in low-rise buildings were the highest among all types of houses.The indoor concentration of 222 Rn had no relation with the building age (F = 0.53,P ＞ 0.05),but that of 220 Rn was dependent on the building age (F = 3.56,P ＜ 0.05).Moreover,the investigation demonstrated indoor 220 Rn concentrations in houses with no decoration were higher than in the houses decorated (t = 2.33,P ＜0.05).The average indoor concentrations of 222Rn and 220Rn in Yuhang district were 32.5 Bq/m3 and 314.3 Bq/m3,respectively,and the annual effective doses were 0.88 mSy and 0.42 mSv respectively.The average indoor concentrations of 222Rn and 220Rn in Sanmen county were 26.8 Bq/m3 and 399.5 Bq/m3,and the annual effective doses were 0.72 mSy and 0.53 mSv respectively.Conclusion The concentrations of indoor 222 Rn in some areas of Zhejiang province are at natural background level,and the concentrations of indoor 220Rn in rural areas are relatively higher.The total annual effective dose from 220Rn and its progeny was larger than that from 222Rn and its progeny by 50 percents.