Abstract: While groundwater flows through an unconfined aquifer due to heterogeneity, the vertical average saturated hydraulic conductivity is a function of hydraulic head. In this study, the MODFLOW model was used to calculate the relation between hydraulic head and hydraulic conductivity in Pingtung Plain. The hydraulic conductivity was transformed with exponential function to examine the distribution properties of fuzzy function. The result shows that the variation of hydraulic conductivity with the changes of groundwater head in observation wells in Pingtung Plain is of fuzzy properties. This analysis indicated that the hydraulic conductivity could not be assumed as a constant value when groundwater flow was simulated with numerical model.