“Three Fu” (TF) is a typical regional climate phenomenon in association with the hottest period in summer in part of China. The traditional concept of the TF has been described as an uncomfortably hottest period in summer, lasting for 30 or 40 days, without a quantified meteorological definition. In order to study this typical monthly-scale extreme climate event, the authors present a quantified definition of the TF characteristis in using the Temperature-Humidity Index (THI) based on the daily minimum/maximum temperatures (Tmin/Tmax) and the relative moisture observations during 1960-2004. Four divisions are categorized in China: TF, quasi-TF, potential-TF, and none-TF areas. The TF area covers most of southeastern China, where Asian summer monsoon prevails. To compare the TF characteristics in different 〖JP〗climatological zones, the TF area is further divided into three subregions: the North China Plain (NC), the YangtzeHuaihe River basins (YH), and the areas south of the Yangtze River (SY). The beginning dates of the TF are mainly between 4 and 25 July, but on average, around 16 July in NC (lasting for 26 days), 16 July in YH (lasting for 29 days), and 13 July in SY (lasting for 34 days). The intensity (in terms of THI) of the TF is the strongest in YH, slightly weaker in SY, and the weakest in NC. The new TF index based on Tmin exhibits an enhancing trend during 1960-2004, but that based on Tmax is of strong interdecadal variations.