Hydrologic drought is a common natural phenomenon. Its essential properties and pertinent indexes are important topics in drought study and hydrologic analysis and calculation, and also are important bases for planning and management of water resources system. In the view of the fact that the quantitative investigation to hydrologic drought is very limited in China, the definition of hydrologic drought is briefily described. From the viewpoint of hydrology. it is pointed out that hydrologic drought is a low flow process in which the stream flow is successively lower than a given truncation level in any averaging period such as a month, a season. a year. etc. The physical concepts of the main indexes. duration, severity and density. in a hydrologic drought. are explained. Also the basic concept of critical hydrologic drought duration is introduced.Properties of probability distribution of critical hydrologic drought durations in which the drought is characterized by a mean annual stream flow series with statistical independence are investigated. It is widely recognized that the most primary difficulty for quantitative investigation to hydrologic drought is the deficiency in measured data. Two methods are used in this study to solve the difficulty. One is the direct method. namely the analytic or statistic method. another is the indirect mothod, i. e. the experimental or simulative method. A probability density function of critical hydrologic drought durations characterized by the longest negative run lengths is transformed from a probability density function of the longest positive run lengths for an independently and identically distributed random variable series derived by Sen, using the enumeration method. Accordingly, a probability distribution function of critical hydrologic drought durations is also derived. A skewed (Pearson type three) one─order autoregressive model is used to simulate mean annual stream flow series based on both the characteristics of the stream flow in the study region and the experience in simulating practice.The region selected for this study is the Wei River basin. the largest tributary of the Yellow River. including two major tributaries of the Wei River. the Jing River and the North Luo River, and each of the three rivers has main station. A 60-years measured mean annual stream flow series for each station is used for the analysis. It is observed that the simulated flow series efficiently preserved the main statistic properties of the measured series. The results show a pretty goodaccordance among the probability densrties and distributions of critical hydrologic drought durations of the theoretical (analytic) model, and the simulated and measured data. In conventional or scientific hydrologic classification of drought and high flows, the mean or median of flow series is commonly regarded as a standard truncation level. However, it is not consistent with the design criterion of water supply projects and is lack of practical value. Therefore, for thepractical purposes, it is suggested that the flow corresponding to the design criterion of water supply projects should be a operational truncation level of hydrologic drought. It makes the drought have a meaning of" Water conservancy drought" . Because the design criterion has obvious regional properties, the classification criterion of hydrologic drought should reflect the regional properties. Therefore, the criterion can be regarded as a regionalized hydrologic drought index or parameter. Critical hydrologic drought duration is an important index in determining the drought protection period of a water supply project in the planning of water resources system. However, during a certain period, such as the economic life or safe service life of a project, the duration of critical hydrologic drought may be long or short but with different recurrent frequencies. If a relative long duration is used as the drought protection period. it may lead to a larger project scale and may be extravagant in funds due to the less recurrent opportunities. On the