In this paper, the geographical and seasonal distributions of the wet and dry periods and their influences on the agriculture of China are discussed. By using the meteorological data during 1951-60 of 310 stations, the durations of the wet and dry periods are de- termined by the histogram of the monthly aridity. The results of the calculations (Fig. 3-8) are in good agreement with facts. Based on the indices of aridity, they are distinguished into the moist, humid, arid and dry periods. The indices of aridity(D) are obtained from the following formula: D=Σd/r，where Σd is the sum of the daily saturation deficit(mb) and r the monthly precipitation (mm). The criteria for wet and dry periods are shown in table 1.