Abstract: The joint probability behaviors of extreme hydro-meteorological events, such as droughts, have been receiving increasing attentions in recent years. Since extreme hydro-meteorological events are reflected by more than one variable, such as duration and intensity, copula functions have been widely applied. However, uncertainties of copula-based analysis of hydrological droughts as a result of selection of marginal distribution and copulas have not yet received significant concerns. The aim of this study is to evaluate such uncertainties based on Bayesian approach. The method is used to analyze hydrological drought in the East River basin (China), which is the principal supplier of water resources for megacities in the Pearl River Delta and also for Hong Kong. The results indicate that the credible intervals of most likely design drought events with a return period of 20 years in terms of drought severity and duration are considerably large at all stations in the East River basin, covering the drought event curves at return periods of 10 and 50 years. Also the influences of heavy-tailed marginal distribution on the uncertainty evaluation of joint distribution have been investigated. Results show that the stronger the heavy-tailed marginal distribution, the greater the uncertainty of the joint distribution, especially for the extreme event. Results of this study provide a technical reference for uncertainty evaluation in copula-based analysis of drought events at regional and global scales. The large credible interval of drought severity and duration greatly challenges measures of mitigation of drought hazards and water resource management.