|Effective is a potential source of water resource for irrigation, the estimation and utilization of effective rainfall are predominantly based on the irrigation area of reservoir systems in Taiwan. The effective rainfall is always previous estimated and listed in the annual irrigation scheduling of each Irrigation Associations in Taiwan, It is used to forecast the potential available rainfall for irrigation and conducted to deduct the irrigation water in the next year. But the irrigation area of non-reservoir is de-emphasis on the utilization of the natural rainfall. Whereas the water resources are increasingly difficult to develop and deploy in Taiwan, the appropriate usage of effective rainfall will be helpful to improve the whole water resource problems which of the effective rainfall is regarded as one source of irrigation water. In this study, Taoyuan, Taichung, Kaohsiung and Taitung irrigation areas are selected as the research region. Water gate operation method and the continuous probability distribution methods are conducted to evaluate the effective rainfall and planned effective rainfall, respectively. The 20-year long daily rainfall data in the period of 198 1-2000 are used to estimate the rainfall characteristics, utilization rate and use efficiency of effective rainfall in the northern, middle, southern, and eastern of TaiwanThe result shows that the average of annual effective rainfall is 1,005 mm/yr, 802 mm/yr, 809 mm/yr, and 827 mm/yr respectively in Taoyuan, Taichung, Kaohsiung, and Taitung irrigation areas. The corresponding utilization rate of annual effective rainfall is 54.9%, 53.6%, 41.0%, and 49.6%, respectively. Further, the continuous probability distribution method was conducted to estimate the monthly planned effective rainfall of 53 workstations from January to December. According to the result of 636 times of mathematical calculation, it shows that the optimal distribution of monthly effective rainfall is primary characterized by Gama distribution, the subsequent models are the normaI distribution, and the fifth least value of the monthly effective rainfall for the past 20 years.