The Gumbel logistic model is used to describe the dependent structure between the flood peak and flood volume and a bivariate joint distribution of peak-volume using PDS sample approach is deve loped. Based on the derived bivariate distribution, the approaches for calculating conditional frequency and bivariate return period are presented. The Geheyan reservoir is selected as a case study. The assumption that the peak and volume adopt a same frequency in design flood hydrograph is discussed through the conditional frequency of flood volume given flood peak exceeding a flood quantity with a specified frequency. The bivariate return periods of design values for peak and volume in pairs derived from univariate distribution respectively are estimated. The results of the study show that the assumption is not considered to be very conservative but rational in a certain extent.