In general,the soil radon is a major source of indoor radon for the rooms at the first floor of buildings.In order to understand the level and distribution characteristics of 222Rn/220Rn concentrations in soil gas in the high- radiation-background area,222Rn/220Rn survey was carried out for the first time using a RAD7 portable radon monitor at 67 locations covered the total area of 1800 krri2 in Zhongshan City,a southern city located in Guangdong Province,China.220Rn concentrations were significantly different from that in the surface areas covered by weathered granite of Yanshan Period or Quaternary sediments.The 220Rn concentrations varied between 6.65 kBqm.3 and 461 kBqm-3 and the average were 294.42+81.36 kBqm.3 and 23.30+25.84 kBqm-3 for weathered granite products and sediments,respectively.The high positive correlation between 220Rn concentrations and 232Th activity concentrations was found.220Rn concentrations had no statistically significant variations from depths of 20 to 140 cm with an interval of 20 cm.It is worth paying attention to the problem of such a high soil 220Rn concentration in Zhuhai City and Zhongshan City.Compared with the contour map of 220Rn,the distribution pattern of 222Rn is similar with the following characteristics.First of all,the sites with 222Rn high-value are mainly located in the granite outcrops,but the areas with high values are relatively small.222Rn concentrations are higher in the sites situated in the part of all-weathered granite outcrops than that in the outcrops of semi-weathered granite rock.Secondly,there is a small piece of high 222Rn level occurring in the region of the Quaternary sediments.In general,low levels of 222Rn and 220Rn concentrations are caused by the low porosity and permeability of the moist compact clay soil.However,high levels of 222Rn and 220Rn will be noted when the surface soils overlaying with fresh decomposed granite products.The soil gas 222Rn concentrations will increase as long as the thickness of backfill of weathered granite products with high 226Ra content reaches 100 cm,although these sites are located in Quaternary sediment region.Thirdly,a 222Rn anomaly was found located at the southern part of the surveyed area and its value reached 1199 kBqm.3,located in a small weathered granite outcrops.The next highest concentration (486 kBqm.3) was recorded at a site in the western part of Wuguishan Mountain (WGS).The soil radon survey results in a large scale show that soil gas 222Rn/220Rn concentrations are relatively high in the regions of granite outcrops in Zhongshan City.Their distribution patterns coincide with 232Th specific radioactivity.It may lead to higher levels of indoor 220Rn and its progenies concentrations because specific radioactivity of 226Ra and 232Th in the granitic rocks in Zhongshan City is almost the same as that in Yangjiang City and in Zhuhai City.This research was supported by the Natural Science Foundation of China [No.40174096].