Abstract: Background and Objective: Water Source Security (WSS) is one of the key challenges for many countries due to climate change, limited water supply and growing water demand. The objective of this study was to find out the important factors which influence the security of water sources in the mainstream of the "Da" River, Vietnam. Materials and Methods: In this study, three groups of factors affecting WSS were considered including, (1) Policies and mechanisms factors, (2) Water demand factors and (3) Natural factors. Fuzzy analytic hierarchy process method was applied to quantify the factors which influence WSS. The input data were collected by means of face-to-face interviews with water resources experts. Results: The results show that the group of natural factors were the most affecting group to water security, followed by the group of mechanism and policy factors and group of water demand factors. Conclusion: This study has found out the most affecting group to WSS using the fuzzy AHP approach. The results obtained from this study may serve as a reference for policy makers in water resources management.